[compiler-rt] Always use C++17 when building compiler-rt standalone

[glandium]

Summary

Platforms where -std=c++17 is not a valid flag (e.g. Windows with
clang-cl) are currently not getting the flag. This makes the setup for
compiler-rt similar to other subprojects.

[glandium]

@smeenai I see you pushed something similar in 5737f6a527d782e6577e5cc1e0c743df2c98546a, but missing the config-ix.cmake part, would you mind pushing that cleanup too?

[smeenai]

Ah, sorry, I didn't realize you already had this change up. I pushed rGd7a4ff9986c8dcaf967770c868e9020d3f328e9f.

[glandium]

Thank you

compiler-rt/CMakeLists.txt

 endif()
 set(COMPILER_RT_HWASAN_WITH_INTERCEPTORS ${COMPILER_RT_HWASAN_WITH_INTERCEPTORS_DEFAULT} CACHE BOOL "Enable libc interceptors in HWASan (testing mode)")
 
 set(COMPILER_RT_BAREMETAL_BUILD OFF CACHE BOOL
   "Build for a bare-metal target.")
 
 if (COMPILER_RT_STANDALONE_BUILD)
   load_llvm_config()
+  set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to conform to")
+  set(CMAKE_CXX_STANDARD_REQUIRED YES)
+  set(CMAKE_CXX_EXTENSIONS NO)
   if (TARGET intrinsics_gen)
     # Loading the llvm config causes this target to be imported so place it
     # under the appropriate folder in an IDE.
     set_target_properties(intrinsics_gen PROPERTIES FOLDER "Compiler-RT Misc")
   endif()
 
   find_package(Python3 COMPONENTS Interpreter)
   if(NOT Python3_Interpreter_FOUND)

compiler-rt/cmake/config-ix.cmake

 check_cxx_compiler_flag(-fomit-frame-pointer COMPILER_RT_HAS_FOMIT_FRAME_POINTER_FLAG)
 check_cxx_compiler_flag(-funwind-tables      COMPILER_RT_HAS_FUNWIND_TABLES_FLAG)
 check_cxx_compiler_flag(-fno-stack-protector COMPILER_RT_HAS_FNO_STACK_PROTECTOR_FLAG)
 check_cxx_compiler_flag(-fno-sanitize=safe-stack COMPILER_RT_HAS_FNO_SANITIZE_SAFE_STACK_FLAG)
 check_cxx_compiler_flag(-fvisibility=hidden  COMPILER_RT_HAS_FVISIBILITY_HIDDEN_FLAG)
 check_cxx_compiler_flag(-frtti               COMPILER_RT_HAS_FRTTI_FLAG)
 check_cxx_compiler_flag(-fno-rtti            COMPILER_RT_HAS_FNO_RTTI_FLAG)
 check_cxx_compiler_flag("-Werror -fno-function-sections" COMPILER_RT_HAS_FNO_FUNCTION_SECTIONS_FLAG)
-check_cxx_compiler_flag(-std=c++17           COMPILER_RT_HAS_STD_CXX17_FLAG)
 check_cxx_compiler_flag(-ftls-model=initial-exec COMPILER_RT_HAS_FTLS_MODEL_INITIAL_EXEC)
 check_cxx_compiler_flag(-fno-lto             COMPILER_RT_HAS_FNO_LTO_FLAG)
 check_cxx_compiler_flag(-fno-profile-generate COMPILER_RT_HAS_FNO_PROFILE_GENERATE_FLAG)
 check_cxx_compiler_flag(-fno-profile-instr-generate COMPILER_RT_HAS_FNO_PROFILE_INSTR_GENERATE_FLAG)
 check_cxx_compiler_flag(-fno-profile-instr-use COMPILER_RT_HAS_FNO_PROFILE_INSTR_USE_FLAG)
 check_cxx_compiler_flag(-fno-coverage-mapping COMPILER_RT_HAS_FNO_COVERAGE_MAPPING_FLAG)
 check_cxx_compiler_flag("-Werror -mcrc32"    COMPILER_RT_HAS_MCRC32_FLAG)
 check_cxx_compiler_flag("-Werror -msse3" COMPILER_RT_HAS_MSSE3_FLAG)