verify-di: Call debug info verifier from clang

This is paired with a patch to LLVM that creates a separate pass for
verifying debug info.

<rdar://problem/15500563>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206301 91177308-0d34-0410-b5e6-96231b3b80d8