clang-cl: Support /showIncludes

This option prints information about #included files to stderr. Clang could
already do it, this patch just teaches the existing code about the /showIncludes
style and adds the flag.

Differential Revision: http://llvm-reviews.chandlerc.com/D1333

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188037 91177308-0d34-0410-b5e6-96231b3b80d8