[PDB] Resubmit "Support embedding natvis files in PDBs."

[PDB] Resubmit "Support embedding natvis files in PDBs."

This was reverted several times due to what ultimately turned out
to be incompatibilities in our serialized hash table format.

Several changes went in prior to this to fix those issues since
they were more fundamental and independent of supporting injected
sources, so now that those are fixed this change should hopefully
pass.

llvm-svn: 328363