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

This reverts commit r187991 and adjusts the comment.  /Za is much more
involved, and we don't want to give anyone the impression we actually
support it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187998 91177308-0d34-0410-b5e6-96231b3b80d8