support: clean up MSVC support

Visual Studio 2013 (CRT version 12) added support for many C99 long long and
long double functions. Visual Studio 2015 (CRT version 14) increased C99 and C11
compliance further. Since we don't support Visual Studio versions older than
2013, we can considerably clean up the support header.

Patch by Shoaib Meenai!

llvm-svn: 280988