Revert r263974, "clang-cl: With -fmsc-version=1900, use MSVS2015 diag formatting."

It seems the test wouldn't expect if default target is *-win32.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@264007 91177308-0d34-0410-b5e6-96231b3b80d8