Add debug location verification for !llvm.loop attachments.
This patch teaches the Verifier how to detect broken !llvm.loop
attachments as discussed in https://reviews.llvm.org/D60831. This
allows LLVM to warn and strip out the broken debug info before
attempting an LTO compilation with input generated by LLVM predating
Differential Revision: https://reviews.llvm.org/D63499
[Re-applies r363725 without changes after fixing a broken testcase.]