[PDB] Put LLD's PDB writing code in a class NFC

Summary:
We've accumulated about five or so data structures that are widely
referenced:
- PDBBuilder
- Type table
- Id table
- PDB string table
- Type server handler

I'm about to rewrite type server handling, and I need a new class in LLD
where I can put its state. By creating a new PDBLinker class, I hope to
put it there next.

Reviewers: ruiu

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35392

llvm-svn: 307979