clang-cl: Hook up /Za to prevent adding oldnames.lib dependency

The /Za flag should probably do more, but let's start with this.

Differential Revision: http://llvm-reviews.chandlerc.com/D1320

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187991 91177308-0d34-0410-b5e6-96231b3b80d8