[compiler-rt][NFC] Apply -nostdinc++ only to C++ source in profile runtime

Avoid passing the C++-specific `-nostdinc++` flag to C source files in
the profile runtime library, which triggers warnings when building with GCC:

  cc1: warning: command-line option ‘-nostdinc++’ is valid for C++/ObjC++ but not for C

We only need `-nostdinc++` for `InstrProfilingRuntime.cpp` to prevent accidental
inclusion of the C++ standard library headers. This patch scopes the flag to
that file using `set_source_files_properties()` and removes the flag from
the global `EXTRA_FLAGS`.

Author: ReVe1uv

compiler-rt/lib/profile/CMakeLists.txt

@@ -142,7 +142,13 @@ if(MSVC)
 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)
+if(COMPILER_RT_HAS_NOSTDINCXX_FLAG)
+  set_source_files_properties(
+    InstrProfilingRuntime.cpp
+    PROPERTIES
+      COMPILE_FLAGS "-nostdinc++"
+  )
+endif()
 # XRay uses C++ standard library headers.
 string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")