Fix use after free in PDB linker.

When merging in types from a type server PDB, we would use a
pointer into the type server PDB's mapped file buffer directly
to avoid copying data.  However, we would close the type server
PDB after we finished merging in its types, which would unmap
all of its memory.  This would lead to a use after free.

We fix this by making a strong reference in the PDBLinker class
to all referenced type server PDBs, thereby making it safe to
hold pointers into its memory mapped contents.

This fixes llvm.org/pr36455

Differential Revision: https://reviews.llvm.org/D43834

llvm-svn: 326345