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.
Needs a --target=aarch64 or similar to use sve.