Since some time clang itself figures out the default for...

Since some time clang itself figures out the default for ms-compatibility-version  and uses it. Trying to figure it out during build is redundant and also will not work when the environment variable VSINSTALLDIR is not defined (which is not defined if you don't install whole Visual Studio but use Visual C++ Build Tools package).

Tested by bootstrapping clang with clang-cl.

Differential Revision: http://reviews.llvm.org/D20672

llvm-svn: 270860