[Index] Add various missing USR generation

Over the years there's been many builtin types added without
corresponding USRs. Add a `@BT@<name>` USR for all these types. Also add
a comment so that hopefully this doesn't continue happening.

`MSGuid` was also missing a USR, use `@MG@GUID{<uuid>}` for it.

Resolves rdar://102198268.

Differential Revision: https://reviews.llvm.org/D138322