[pdb] Don't choke on unknown symbol types.

[pdb] Don't choke on unknown symbol types.

When we get an unknown symbol type, we might as well at least
dump it.  Same goes for round-tripping through YAML, we can
dump the record contents as raw bytes even if we don't know
how to interpret it semantically.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305248 91177308-0d34-0410-b5e6-96231b3b80d8