[libc++][CI] Tests the no RTTI configuration. (#65518)

There are a few drive-by fixes:
- Since the combination RTTI disabled and exceptions enabled do not
work, this combination is prohibited.
- A small NFC in any fixing clang-tidy.

The code in the Buildkite configuration is prepared for using the std
module. There are more fixes needed for that configuration which will be
done in a separate commit.

.github/workflows/libcxx-build-and-test.yaml

@@ -160,6 +160,7 @@ jobs:
           'generic-no-tzdb',
           'generic-no-unicode',
           'generic-no-wide-characters',
+          'generic-no-rtti',
           'generic-static',
           'generic-with_llvm_unwinder',
           # TODO Find a better place for the benchmark and bootstrapping builds to live. They're either very expensive

libcxx/CMakeLists.txt

@@ -284,7 +284,9 @@ endif()
 
 # Feature options -------------------------------------------------------------
 option(LIBCXX_ENABLE_EXCEPTIONS "Use exceptions." ON)
-option(LIBCXX_ENABLE_RTTI "Use run time type information." ON)
+option(LIBCXX_ENABLE_RTTI
+  "Use runtime type information.
+   This option may only be set to OFF when LIBCXX_ENABLE_EXCEPTIONS=OFF." ON)
 option(LIBCXX_ENABLE_THREADS "Build libc++ with support for threads." ON)
 option(LIBCXX_ENABLE_MONOTONIC_CLOCK
   "Build libc++ with support for a monotonic clock.
@@ -374,6 +376,13 @@ if (LIBCXX_HAS_PTHREAD_API)
   endif()
 endif()
 
+if (NOT LIBCXX_ENABLE_RTTI AND LIBCXX_ENABLE_EXCEPTIONS)
+  message(FATAL_ERROR "Libc++ cannot be built with exceptions enabled but RTTI"
+                      " disabled, since that configuration is broken. See"
+                      " https://github.com/llvm/llvm-project/issues/66117"
+                      " for details.")
+endif()
+
 # Ensure LLVM_USE_SANITIZER is not specified when LIBCXX_GENERATE_COVERAGE
 # is ON.
 if (LLVM_USE_SANITIZER AND LIBCXX_GENERATE_COVERAGE)

libcxx/cmake/caches/Generic-no-rtti.cmake

@@ -0,0 +1,4 @@
+set(LIBCXX_ENABLE_RTTI OFF CACHE BOOL "")
+set(LIBCXX_ENABLE_EXCEPTIONS OFF CACHE BOOL "")
+set(LIBCXXABI_ENABLE_RTTI OFF CACHE BOOL "")
+set(LIBCXXABI_ENABLE_EXCEPTIONS OFF CACHE BOOL "")

libcxx/docs/BuildingLibcxx.rst

@@ -369,6 +369,7 @@ libc++ Feature Options
   **Default**: ``ON``
 
   Build libc++ with run time type information.
+  This option may only be set to OFF when LIBCXX_ENABLE_EXCEPTIONS=OFF.
 
 .. option:: LIBCXX_INCLUDE_TESTS:BOOL
 

libcxx/utils/ci/run-buildbot

@@ -472,6 +472,11 @@ generic-no-exceptions)
     check-runtimes
     check-abi-list
 ;;
+generic-no-rtti)
+    clean
+    generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-no-rtti.cmake"
+    check-runtimes
+;;
 #
 # Other miscellaneous jobs
 #