[PDB] Don't crash on /debug:fastlink PDBs.

Apparently support for /debug:fastlink PDBs isn't part of the
DIA SDK (!), and it was causing llvm-pdbdump to crash because
we weren't checking for a null pointer return value.  This
manifests when calling findChildren on the IDiaSymbol, and
it returns E_NOTIMPL.

llvm-svn: 304982