Add support for Bazel builds on Windows with clang-cl.

Adds basic `--config=clang-cl` to set up the basic options needed, and
then fix a number of issues that surface in Windows builds for me.

With these fixes, `//llvm/...` builds cleanly. One unittest still fails,
but its just due to running out of stack space due to creating a large
number of short-lived stack variables. The test should probably be
decomposed into a set of tests (`LegalizerInfoTest::RuleSets`), but that
seemed like too invasive of a change here and with everything building
cleanly this isn't disrupting me experimenting with Windows builds.

Some parts of `//clang/...` builds, but that will require more work.

utils/bazel/.bazelrc

@@ -69,9 +69,18 @@ build:generic_gcc --copt=-Wno-misleading-indentation --host_copt=-Wno-misleading
 build:generic_gcc --copt=-Werror --host_copt=-Werror
 
 ###############################################################################
-# Windows specific flags for building with VC.
+# Generic Windows flags common to both MSVC and Clang.
 ###############################################################################
 
+# Yay for security warnings. Boo for non-standard.
+build:windows --copt=/D_CRT_SECURE_NO_WARNINGS --host_copt=/D_CRT_SECURE_NO_WARNINGS
+
+###############################################################################
+# Windows specific flags for building with MSVC.
+###############################################################################
+
+build:msvc --config=windows
+
 build:msvc --copt=/WX --host_copt=/WX    # Treat warnings as errors...
 # ...but disable the ones that are violated
 build:msvc --copt=/wd4141 --host_copt=/wd4141 # inline used more than once
@@ -87,8 +96,30 @@ build:msvc --linkopt=/WX --host_linkopt=/WX # Treat warnings as errors...
 # ...but disable the ones that are violated.
 build:msvc --linkopt=/IGNORE:4001 --host_linkopt=/IGNORE:4001 # no object files
 
-# Yay for security warnings. Boo for non-standard.
-build:msvc --copt=/D_CRT_SECURE_NO_WARNINGS --host_copt=/D_CRT_SECURE_NO_WARNINGS
+###############################################################################
+# Options for Windows `clang-cl` builds.
+###############################################################################
+
+# We just start with the baseline Windows config as `clang-cl` doesn't accept
+# some of the generic Clang flags.
+build:clang-cl --config=windows
+
+# Switch from MSVC to the `clang-cl` compiler.
+build:clang-cl --compiler=clang-cl
+
+# C++14 standard version is required.
+build:clang-cl --cxxopt=/std:c++14 --host_cxxopt=/std:c++14
+
+# Use Clang's internal warning flags instead of the ones that sometimes map
+# through to MSVC's flags.
+build:clang-cl --copt=/clang:-Wall --host_copt=/clang:-Wall
+build:clang-cl --copt=/clang:-Werror --host_copt=/clang:-Werror
+
+# This doesn't appear to be enforced by any upstream bot.
+build:clang-cl --copt=/clang:-Wno-unused --host_copt=/clang:-Wno-unused
+
+# There appears to be an unused constant in GoogleTest on Windows.
+build:clang-cl --copt=/clang:-Wno-unused-const-variable --host_copt=/clang:-Wno-unused-const-variable
 
 ###############################################################################
 

utils/bazel/llvm-project-overlay/clang/BUILD.bazel

@@ -29,7 +29,6 @@ cc_binary(
     copts = [
         "$(STACK_FRAME_UNLIMITED)",
     ],
-    linkopts = ["-ldl"],
     stamp = 0,
     deps = [
         "//llvm:Support",

utils/bazel/llvm-project-overlay/llvm/BUILD.bazel

@@ -3102,7 +3102,9 @@ cc_binary(
     name = "llvm-mca",
     srcs = glob([
         "tools/llvm-mca/*.cpp",
+        "tools/llvm-mca/*.h",
         "tools/llvm-mca/Views/*.cpp",
+        "tools/llvm-mca/Views/*.h",
     ]),
     copts = llvm_copts,
     stamp = 0,

utils/bazel/llvm-project-overlay/llvm/config.bzl

@@ -25,15 +25,17 @@ posix_defines = [
     r'LTDL_SHLIB_EXT=\".so\"',
     r'LLVM_PLUGIN_EXT=\".so\"',
     "LLVM_ENABLE_THREADS=1",
-    "HAVE_SYSEXITS_H=1",
-    "HAVE_UNISTD_H=1",
-    "HAVE_STRERROR_R=1",
+    "HAVE_DEREGISTER_FRAME=1",
     "HAVE_LIBPTHREAD=1",
     "HAVE_PTHREAD_GETNAME_NP=1",
-    "HAVE_PTHREAD_SETNAME_NP=1",
     "HAVE_PTHREAD_GETSPECIFIC=1",
+    "HAVE_PTHREAD_H=1",
+    "HAVE_PTHREAD_SETNAME_NP=1",
     "HAVE_REGISTER_FRAME=1",
-    "HAVE_DEREGISTER_FRAME=1",
+    "HAVE_SETENV_R=1",
+    "HAVE_STRERROR_R=1",
+    "HAVE_SYSEXITS_H=1",
+    "HAVE_UNISTD_H=1",
 ]
 
 linux_defines = posix_defines + [

utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/config.h

@@ -174,7 +174,7 @@
 #define HAVE_PTHREAD_GETSPECIFIC 1
 
 /* Define to 1 if you have the <pthread.h> header file. */
-#define HAVE_PTHREAD_H 1
+/* HAVE_PTHREAD_H defined in Bazel */
 
 /* Have pthread_mutex_lock */
 #define HAVE_PTHREAD_MUTEX_LOCK 1
@@ -186,7 +186,7 @@
 /* HAVE_SBRK defined in Bazel */
 
 /* Define to 1 if you have the `setenv' function. */
-#define HAVE_SETENV 1
+/* HAVE_SETENV defined in Bazel */
 
 /* Define to 1 if you have the `setrlimit' function. */
 #define HAVE_SETRLIMIT 1

utils/bazel/llvm-project-overlay/llvm/unittests/BUILD.bazel

@@ -112,7 +112,7 @@ cc_library(
 
 cc_test(
     name = "codegen_tests",
-    size = "small",
+    size = "medium",
     srcs = glob(
         [
             "CodeGen/*.cpp",
@@ -167,7 +167,7 @@ cc_test(
 
 cc_test(
     name = "debuginfo_tests",
-    size = "small",
+    size = "medium",
     srcs = glob(
         [
             "DebugInfo/DWARF/*.cpp",
@@ -219,7 +219,7 @@ cc_test(
 
 cc_test(
     name = "execution_engine_mcjit_tests",
-    size = "small",
+    size = "medium",
     srcs = glob(
         [
             "ExecutionEngine/MCJIT/*.cpp",
@@ -245,7 +245,7 @@ cc_test(
 
 cc_test(
     name = "execution_engine_orc_tests",
-    size = "small",
+    size = "medium",
     srcs = glob(
         [
             "ExecutionEngine/Orc/*.cpp",
@@ -381,7 +381,7 @@ cc_test(
 
 cc_test(
     name = "mi_tests",
-    size = "small",
+    size = "medium",
     srcs = glob(
         ["MI/*.cpp"],
         allow_empty = False,
@@ -525,9 +525,10 @@ cc_test(
     ],
 )
 
+# Note that the name of this test is important as it is used in the test itself.
 cc_test(
-    name = "support_tests",
-    size = "small",
+    name = "SupportTests",
+    size = "medium",
     srcs = glob(
         [
             "Support/*.cpp",