Try to fix MSVS build after r222384. No intended behavior change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222386 91177308-0d34-0410-b5e6-96231b3b80d8
ADT: windows itanium targets msvcrt.dll

Update the isOSMSVCRT to correctly identify that windows-itanium uses
msvcrt.dll.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222389 91177308-0d34-0410-b5e6-96231b3b80d8