[PDB] Add symbols to the PDB

Summary:
The main complexity in adding symbol records is that we need to
"relocate" all the type indices. Type indices do not have anything like
relocations, an opaque data structure describing where to find existing
type indices for fixups. The linker just has to "know" where the type
references are in the symbol records. I added an overload of
`discoverTypeIndices` that works on symbol records, and it seems to be
able to link the standard library.

Reviewers: zturner, ruiu

Subscribers: llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D34432

llvm-svn: 305933