Verifier: Verify that each inlinable callsite of a debug-info-bearing function

in a debug-info-bearing function has a debug location attached to it. Failure to
do so causes an "!dbg attachment points at wrong subprogram for function"
assertion failure when the inliner sets up inline scope info.

rdar://problem/25878916

This reaplies r267320 without changes after fixing an issue in the OpenMP IR
generator in clang.

llvm-svn: 267370