DWARF type hashing: begin implementing Step 5, summary hashing in declarable contexts

There are several other tag types that need similar handling but to
ensure test coverage they'll be coming incrementally.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193126 91177308-0d34-0410-b5e6-96231b3b80d8