[PDB] Ignore all S_UDT symbols when writing PDBs.

We don't have the right algorithm for copying S_UDT symbols
from object files to the globals stream, and having it wrong
is worse than not having it at all, since it breaks display
of local variables of UDT types (for example, "dv Foo" fails
in our current implementation, but succeeds if the S_UDT records
are omitted).  Omit them until we fix the algorithm.

llvm-svn: 310867