Added isTargetWindowsMSVC(), renamed isTargetMingw() to isTargetWindowsGNU()
and isTargetCygwin() to isTargetWindowsCygwin() to be consistent with the
four Windows environments in Triple.h.

Suggestion by Saleem Abdulrasool!



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205393 91177308-0d34-0410-b5e6-96231b3b80d8