[Verifier] Constrain where DILocations may be nested

Summary:
Constrain which metadata nodes are allowed to be, or contain,
DILocations. This ensures that logic for updating DILocations in a
Module is complete.

Currently, !llvm.loop metadata is the only odd duck which contains
nested DILocations. This has caused problems in the past: some passes
forgot to visit the nested locations, leading to subtly broken debug
info and late verification failures.

If there's a compelling reason for some future metadata to nest
DILocations, we'll need to introduce a generic API for updating the
locations attached to an Instruction before relaxing this check.

Reviewers: aprantl, dsanders

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79245