[codeview] Move type index remapping logic to type merger

Summary:
This removes the 'remapTypeIndices' method on every TypeRecord class. My
original idea was that this would be the beginning of some kind of
generic entry point that would enumerate all of the TypeIndices inside
of a TypeRecord, so that we could write generic graph algorithms for
them without duplicating the knowledge of which fields are type index
fields everywhere. This never happened, and nothing else uses this
method. I need to change the API to deal with merging into IPI streams,
so let's move it into the file that uses it first.

Reviewers: zturner, ruiu

Reviewed By: zturner, ruiu

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D31267

llvm-svn: 298564