[compiler-rt] Use -nostdinc++ in clang_rt.profile to avoid including C++ headers

[ldionne]

Summary

Most of the code in compiler_rt is C code. However, clang_rt.profile
contains the InstrProfilingRuntime.cpp file, which builds as C++. This
means that including e.g. <stdint.h> will actually include libc++'s
<stdint.h> and then #include_next the system's <stdint.h>. However, if
the target we're building compiler-rt for isn't supported by libc++,
this will lead to a failure since libc++'s <stdint.h> includes <__config>,
which performs various checks.

Since the goal seems to *not* be including any header from the C++ Standard
Library in clang_rt.profile, using -nostdinc++ to ensure that doesn't
happen unknowingly seems to make sense.

[ldionne]

I am unsure who's the right people to look at this, so I added some based on $ git shortlog -sne -- compiler-rt/lib/profile.

[dexonsmith]

LGTM.

compiler-rt/cmake/config-ix.cmake

 check_cxx_compiler_flag(/wd4391 COMPILER_RT_HAS_WD4391_FLAG)
 check_cxx_compiler_flag(/wd4722 COMPILER_RT_HAS_WD4722_FLAG)
 check_cxx_compiler_flag(/wd4800 COMPILER_RT_HAS_WD4800_FLAG)
 
 # Symbols.
 check_symbol_exists(__func__ "" COMPILER_RT_HAS_FUNC_SYMBOL)
 
 # Includes.
-check_include_files("sys/auxv.h" COMPILER_RT_HAS_AUXV)
+check_cxx_compiler_flag(-nostdinc++ COMPILER_RT_HAS_NOSTDINCXX_FLAG)
+check_include_files("sys/auxv.h"    COMPILER_RT_HAS_AUXV)
 
 # Libraries.
 check_library_exists(dl dlopen "" COMPILER_RT_HAS_LIBDL)
 check_library_exists(rt shm_open "" COMPILER_RT_HAS_LIBRT)
 check_library_exists(m pow "" COMPILER_RT_HAS_LIBM)
 check_library_exists(pthread pthread_create "" COMPILER_RT_HAS_LIBPTHREAD)
 check_library_exists(execinfo backtrace "" COMPILER_RT_HAS_LIBEXECINFO)
 

compiler-rt/lib/profile/CMakeLists.txt

set(EXTRA_FLAGS
      -fPIC
      -Wno-pedantic)
 endif()
 
 if(COMPILER_RT_TARGET_HAS_ATOMICS)
  set(EXTRA_FLAGS
      ${EXTRA_FLAGS}
      -DCOMPILER_RT_HAS_ATOMICS=1)
-endif() 
+endif()
 
 if(COMPILER_RT_TARGET_HAS_FCNTL_LCK)
  set(EXTRA_FLAGS
      ${EXTRA_FLAGS}
      -DCOMPILER_RT_HAS_FCNTL_LCK=1)
 endif()
 
 if(COMPILER_RT_TARGET_HAS_UNAME)
  set(EXTRA_FLAGS
      ${EXTRA_FLAGS}
      -DCOMPILER_RT_HAS_UNAME=1)
 endif()
 
+# We don't use the C++ Standard Library here, so avoid including it by mistake.
+append_list_if(COMPILER_RT_HAS_NOSTDINCXX_FLAG -nostdinc++ EXTRA_FLAGS)
+
 # This appears to be a C-only warning banning the use of locals in aggregate
 # initializers. All other compilers accept this, though.
 # nonstandard extension used : 'identifier' : cannot be initialized using address of automatic variable
 append_list_if(COMPILER_RT_HAS_WD4221_FLAG /wd4221 EXTRA_FLAGS)
 
 if(APPLE)
   add_compiler_rt_runtime(clang_rt.profile
     STATIC