DebugInfo: Introduce new DIValue, DIETypeSignature to encode references to type units via their signatures

This simplifies type unit and type unit reference creation as well as
setting the stage for inter-type hashing across type unit boundaries.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197539 91177308-0d34-0410-b5e6-96231b3b80d8