[codeview] Add type stream merging prototype

Summary:
This code is intended to be used as part of LLD's PDB writing. Until
that exists, this is exposed via llvm-readobj for testing purposes.

Type stream merging uses the following algorithm:

- Begin with a new empty stream, and a new empty hash table that maps
  from type record contents to new type index.
- For each new type stream, maintain a map from source type index to
  destination type index.
- For each record, copy it and rewrite its type indices to be valid in
  the destination type stream.
- If the new type record is not already present in the destination
  stream hash table, append it to the destination type stream, assign it
  the next type index, and update the two hash tables.
- If the type record already exists in the destination stream, discard
  it and update the type index map to forward the source type index to
  the existing destination type index.

Reviewers: zturner, ruiu

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D20122

llvm-svn: 269521