clang-cl: support -fsyntax-only (PR23197)

This might help running Clang tooling (which appends this option)
with clang-cl command-lines.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@234990 91177308-0d34-0410-b5e6-96231b3b80d8