clang-cl: Add /showFilenames option (PR31957)

Add a /showFilenames option for users who want clang to echo the
currently compiled filename. MSVC does this echoing by default, and it's
useful for showing progress in build systems that doesn't otherwise
provide any progress report, such as MSBuild.

Differential Revision: https://reviews.llvm.org/D52773

llvm-svn: 344234