[libcxx][libcxxabi] Remove the unused CMake checks

[phosek]

Summary

These seemed to have been used in the past but were since replaced
by the add_compile_flags_if_supported function that combine these
these checks and adding the flag, but the original checks were never
removed.

[phosek]

Alternative would be to avoid add_compile_flags_if_supported altogether and instead the combination of check_cxx_compiler_flag and append_if as is used in libunwind (https://github.com/llvm/llvm-project/blob/master/libunwind/CMakeLists.txt#L274). The advantage of the latter is that we could in theory have different config-ix.cmake files (e.g. pre-populated ones for the runtimes build).

[ldionne]

libcxxabi/cmake/config-ix.cmake

52 ↗

(On Diff #201815)

Why are we keeping this one?

[phosek]

libcxxabi/cmake/config-ix.cmake

52 ↗

(On Diff #201815)

It's the only one that's being used in CMakeLists.txt, see https://github.com/llvm/llvm-project/blob/master/libcxxabi/CMakeLists.txt#L264

[phosek]

Does anyone have any opinion on add_compile_flags_if_supported vs check_cxx_compiler_flag + append_if? If not I'm going to land this as is.

[ldionne]

In D62566#1521730, @phosek wrote:

Does anyone have any opinion on add_compile_flags_if_supported vs check_cxx_compiler_flag + append_if? If not I'm going to land this as is.

I have a strong preference for add_compile_flags_if_supported until we have the use case you described above. I'm not saying it's not a good/real use case, just that if we're not taking advantage of it right now, we should just simplify things.

cmake/config-ix.cmake

 " LIBCXX_HAS_COMMENT_LIB_PRAGMA)
   cmake_pop_check_state()
 endif()
 
 if(NOT WIN32 OR MINGW)
   include(CheckLibcxxAtomic)
 endif()
 
-# Check compiler flags
-
-check_cxx_compiler_flag(/WX                     LIBCXX_HAS_WX_FLAG)
-check_cxx_compiler_flag(/WX-                    LIBCXX_HAS_NO_WX_FLAG)
-check_cxx_compiler_flag(/EHsc                   LIBCXX_HAS_EHSC_FLAG)
-check_cxx_compiler_flag(/EHs-                   LIBCXX_HAS_NO_EHS_FLAG)
-check_cxx_compiler_flag(/EHa-                   LIBCXX_HAS_NO_EHA_FLAG)
-check_cxx_compiler_flag(/GR-                    LIBCXX_HAS_NO_GR_FLAG)
-
 # Check libraries
 if(WIN32 AND NOT MINGW)
   # TODO(compnerd) do we want to support an emulation layer that allows for the
   # use of pthread-win32 or similar libraries to emulate pthreads on Windows?
   set(LIBCXX_HAS_PTHREAD_LIB NO)
   set(LIBCXX_HAS_M_LIB NO)
   set(LIBCXX_HAS_RT_LIB NO)
   set(LIBCXX_HAS_SYSTEM_LIB NO)