clang-cl: Parse the /d2Zi+ flag (PR18728)

This is an undocumented, but reportedly widely used flag.
We don't support it, but should be able to parse it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201588 91177308-0d34-0410-b5e6-96231b3b80d8