This patch enables the support for digit separators in C mode in Clang. The digit separators are not supported in C mode but are supported in C++ mode in Clang.
Digit Separators are supported in both C and C++ modes in MS VC/C++ and Intel C/C++ compiler.
To be compatible with MSVC, I have enabled the support for digit separators in C mode in clang.
This is a conforming extension, so please check MicrosoftExt instead of MSVCCompat.