Fix downcasts of unaligned empty/tombstone DenseMap keys for DenseMap<AssertVH<T>, Foo>.

Test Plan: llvm regression test suite

Reviewers: chandlerc, rsmith

Reviewed By: rsmith

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D4976

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217058 91177308-0d34-0410-b5e6-96231b3b80d8