[lld] Fix type server merging with PDBs without IPI stream

PDBs may not necessarily contain an IPI stream. Handle this case
gracefully.

The test case was verified to work with MS link.exe.

Patch by Vladimir Panteleev, with a small simplification

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D63178

llvm-svn: 363213