[PDB] Tweak bad type index error handling

Translate invalid type indices to a sentinel value instead of skipping
the record. Skipping records isn't a good recovery method, because we
can skip a scope open or close record, which will confuse the scope
management code.

We currently have lots of invalid type indices on Microsoft-provided
standard libraries, because the LF_TYPESERVER2 records contain absolute
paths that are only valid on their build servers. Our type server
handlers need to look at other things (GUIDs) to find these type server
PDBs.

llvm-svn: 307726