[cmake] Export driver template to fix standalone build

Export the driver-template.cpp.in file so that tools using
GENERATE_DRIVER work in standalone builds (currently only relevant
for clang). I've given the file an llvm- prefix, as we're now
searching for the file in CMAKE_MODULE_PATH.

Differential Revision: https://reviews.llvm.org/D127384

llvm/cmake/modules/AddLLVM.cmake

@@ -892,9 +892,14 @@ macro(add_llvm_executable name)
 
   if (ARG_GENERATE_DRIVER)
     string(REPLACE "-" "_" TOOL_NAME ${name})
-    configure_file(
-      ${LLVM_MAIN_SRC_DIR}/cmake/driver-template.cpp.in
-      ${CMAKE_CURRENT_BINARY_DIR}/${name}-driver.cpp)
+    foreach(path ${CMAKE_MODULE_PATH})
+      if(EXISTS ${path}/llvm-driver-template.cpp.in)
+        configure_file(
+          ${path}/llvm-driver-template.cpp.in
+          ${CMAKE_CURRENT_BINARY_DIR}/${name}-driver.cpp)
+        break()
+      endif()
+    endforeach()
 
     list(APPEND ALL_FILES ${CMAKE_CURRENT_BINARY_DIR}/${name}-driver.cpp)
 

llvm/cmake/modules/CMakeLists.txt

@@ -173,7 +173,8 @@ if (NOT LLVM_INSTALL_TOOLCHAIN_ONLY)
     PATTERN LLVMConfigExtensions.cmake EXCLUDE
     PATTERN LLVMConfigVersion.cmake EXCLUDE
     PATTERN LLVM-Config.cmake EXCLUDE
-    PATTERN GetHostTriple.cmake EXCLUDE)
+    PATTERN GetHostTriple.cmake EXCLUDE
+    PATTERN llvm-driver-template.cpp.in)
 
   if (NOT LLVM_ENABLE_IDE)
     # Add a dummy target so this can be used with LLVM_DISTRIBUTION_COMPONENTS