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
https://reviews.llvm.org/rL361149.

rdar://problem/51631158

Differential Revision: https://reviews.llvm.org/D63499

[Re-applies r363725 without changes after fixing a broken testcase.]

llvm-svn: 363731