[codeview] Don't use the type visitor to merge types

Summary:
This didn't do much to speed things up, but it implements a FIXME, and I
think it's a nice simplification. We don't need the record kind switch.
We're doing that ourselves.

Reviewers: ruiu, inglorion

Subscribers: llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D35496

llvm-svn: 308213