[codeview] Add a type visitor to help abstract away type stream handling

Summary:
Port the dumper in llvm-readobj over to it.

I'm planning to use this visitor to power type stream merging.

While we're at it, try to switch from StringRef to ArrayRef<uint8_t> in some
places.

Reviewers: zturner, amccarth

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D19899

llvm-svn: 268535