Update msbuild integration warnings: Don't warn on /Zi and /X

We do need to map /Zi to /Z7 explicitly for msbuild as explained in this file,
but since /Zi is passed by default and since things transparently work fine
with it mapped to /Z7, we shouldn't produce effectively inactionable noise for
it.

Also don't warn on /X since clang-cl supports that (since r326357; the risk of
duplicating a bunch of clang-cl driver logic here).

https://reviews.llvm.org/D50398

llvm-svn: 339169