[compiler-rt] Don't explicitly set CMAKE_CXX_FLAGS.

[danalbert]

Summary

C++ flags should not be used for not-C++ files as it may trigger
-Werror=unused-command-line-argument. CMake will use CMAKE_C_FLAGS,
CMAKE_CXX_FLAGS, and CMAKE_ASM_FLAGS as appropriate implicitly, so
this does not need to be explicitly handled here.

This change depends on https://reviews.llvm.org/D53301, since one of
the builders depended on this behavior because it was not configuring
CMAKE_ASM_FLAGS.

[eugenis]

LGTM

compiler-rt/trunk/cmake/Modules/AddCompilerRT.cmake

foreach(libname ${libnames})
 
     # Strip out -msse3 if this isn't macOS.
     set(target_flags ${LIB_CFLAGS})
     if(APPLE AND NOT "${libname}" MATCHES ".*\.osx.*")
       list(REMOVE_ITEM target_flags "-msse3")
     endif()
 
     set_target_compile_flags(${libname}
-      ${CMAKE_CXX_FLAGS} ${extra_cflags_${libname}} ${target_flags})
+      ${extra_cflags_${libname}} ${target_flags})
     set_property(TARGET ${libname} APPEND PROPERTY
       COMPILE_DEFINITIONS ${LIB_DEFS})
     set_target_properties(${libname} PROPERTIES FOLDER "Compiler-RT Libraries")
     if(APPLE)
       set_target_properties(${libname} PROPERTIES
         OSX_ARCHITECTURES "${LIB_ARCHS_${libname}}")
     endif()
   endforeach()