X86Subtarget.h: isTargetWindows() should tell whether he is targeting msvc.

FYI, !isWindowsGNUEnvironment() is insufficient. It missed cygwin.

FIXME: The name "isTargetWindows" should be fixed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205124 91177308-0d34-0410-b5e6-96231b3b80d8