DWARFVerifier: Enhance validation of .debug_names hash tables

Summary:
This patch adds more checks to the .debug_names validator. Specifically,
they check for:
- buckets claiming to be non-empty but pointing to mismatched hashes
  (most consumers would interpret this as an empty bucket, but it
  questionable whether the generator meant that)
- hashes that are not reachable from any bucket
- names with incorrect hashes

Together, these checks ensure that any name in the index can be reached
through the hash table using the regular lookup algorithm. We also warn
if we encounter a name index without a hash table.

Reviewers: JDevlieghere, aprantl, dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D44433

llvm-svn: 327699