Fix UBSan error reports in ValueMapCallbackVH and AssertingVH<T> empty/tombstone keys generation.
Summary:
One more attempt to fix UBSan reports: make sure DenseMapInfo::getEmptyKey()
and DenseMapInfo::getTombstoneKey() doesn't do any upcasts/downcasts to/from Value*.
Test Plan: check-llvm test suite with/without UBSan bootstrap
Reviewers: chandlerc, dexonsmith
Subscribers: llvm-commits, majnemer
Differential Revision: http://reviews.llvm.org/D6903
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225558
91177308-0d34-0410-b5e6-
96231b3b80d8