[PDB] Merge in types and items from type servers (/Zi)

Summary:
Object files compiled with /Zi emit type information into a type server
PDB. The .debug$S section will contain a single TypeServer2Record with
the absolute path and GUID of the type server. LLD needs to load the
type server PDB and merge all types and items it finds in it into the
destination PDB.

Depends on D35495

Reviewers: ruiu, inglorion

Subscribers: zturner, llvm-commits

Differential Revision: https://reviews.llvm.org/D35504

llvm-svn: 308235