Change layering warning tests to not actually build modules.

Specifically, we want to warn only for direct layering violations for
the modules we are calling clang on.

This temporarily unblocks
http://llvm-reviews.chandlerc.com/D2374

Once that is in, we'll also want to investigate whether to check the
layering in the build step of modules that we build transitively.

llvm-svn: 197021