clang-cl: implement custom search for cl.exe in /fallback mode

This solves the problem of fallback onto ourselves if clang-cl
has been renamed to cl.exe and put on the PATH, as happens with
the VS integration.

Differential Revision: http://llvm-reviews.chandlerc.com/D1731

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191099 91177308-0d34-0410-b5e6-96231b3b80d8