Driver: rename Windows to MSVCToolChain

This renames the Windows toolchain to MSVCToolChain.  This is a preparatory step
for adding a CrossWindowsToolChain which uses clang/libc++/(ld/lld) without the
standard GCC toolchain lookup.  NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220362 91177308-0d34-0410-b5e6-96231b3b80d8