[OpenMP] Compile assembly files as ASM, not C

[mstorsjo]

Summary

Since CMake 3.20, CMake explicitly passes "-x c" (or equivalent)
when compiling a file which has been set as having the language
C. This behaviour change only takes place if "cmake_minimum_required"
is set to 3.20 or newer, or if the policy CMP0119 is set to new.

Attempting to compile assembly files with "-x c" fails, however
this is workarounded in many cases, as OpenMP overrides this with
"-x assembler-with-cpp", however this is only added for non-Windows
targets.

Thus, after increasing cmake_minimum_required to 3.20, this breaks
compiling the GNU assembly for Windows targets; the GNU assembly is
used for ARM and AArch64 Windows targets when building with Clang.
This patch unbreaks that.

[mstorsjo]

Ping, this is needed to unbreak my nightly builds

[tianshilei1992]

LG

openmp/CMakeLists.txt

 cmake_minimum_required(VERSION 3.20.0)
 
 set(LLVM_COMMON_CMAKE_UTILS ${CMAKE_CURRENT_SOURCE_DIR}/../cmake)
 
 # Add path for custom modules
 list(INSERT CMAKE_MODULE_PATH 0
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
   "${LLVM_COMMON_CMAKE_UTILS}/Modules"
   )
 
 # llvm/runtimes/ will set OPENMP_STANDALONE_BUILD.
 if (OPENMP_STANDALONE_BUILD OR "${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_SOURCE_DIR}")
   set(OPENMP_STANDALONE_BUILD TRUE)
-  project(openmp C CXX)
+  project(openmp C CXX ASM)
 endif()
 
 # Must go below project(..)
 include(GNUInstallDirs)
 
 if (OPENMP_STANDALONE_BUILD)
   # CMAKE_BUILD_TYPE was not set, default to Release.
   if (NOT CMAKE_BUILD_TYPE)

openmp/runtime/src/CMakeLists.txt

 libomp_append(LIBOMP_SOURCE_FILES libomp.rc WIN32)
 
 # Get compiler and assembler flags
 libomp_get_cxxflags(LIBOMP_CONFIGURED_CXXFLAGS)
 libomp_get_asmflags(LIBOMP_CONFIGURED_ASMFLAGS)
 # Set the compiler flags for each type of source
 set_source_files_properties(${LIBOMP_CXXFILES} PROPERTIES COMPILE_FLAGS "${LIBOMP_CONFIGURED_CXXFLAGS}")
 set_source_files_properties(${LIBOMP_ASMFILES} ${LIBOMP_GNUASMFILES} PROPERTIES COMPILE_FLAGS "${LIBOMP_CONFIGURED_ASMFLAGS}")
-# Let the compiler handle the GNU assembly files
-set_source_files_properties(${LIBOMP_GNUASMFILES} PROPERTIES LANGUAGE C)
 
 # Remove any cmake-automatic linking of the standard C++ library.
 # We neither need (nor want) the standard C++ library dependency even though we compile c++ files.
 if(NOT ${LIBOMP_USE_STDCPPLIB})
   set(LIBOMP_LINKER_LANGUAGE C)
   set(CMAKE_CXX_IMPLICIT_LINK_LIBRARIES)
 else()
   set(LIBOMP_LINKER_LANGUAGE CXX)