The main goal of this is to collapse the LocId and SemIRLoc types into a
single type, eliminating the need for APIs to decide which to use. This
originated from discussion about UnwrapSemIRLoc in #5169. Although that
was removed in #5202, it's probably still a good direction for LocId.
This changes the packing of LocId to allow adding InstId, making it
tri-modal: ImportIRInstId, InstId, or NodeId. This has a side-effect of
reducing the available space for ImportIRInstId, although not by much
due to the pre-existing `ImplicitBit` behavior. If needed, we could also
probably play with packing a bit more since `ImplicitBit` really only
applies to `NodeId`, but I was trying to keep the logic a little
simpler. Note `TokenOnlyBit` can still apply to `ImportIRInstId`.
This leaves in place a typedef for SemIRLoc -- I intend to clean that up
separately.
Some Discord discussion is
[here](https://discord.com/channels/655572317891461132/655578254970716160/1353755830058745959).
This is trying to document the status quo. Note
https://github.com/carbon-language/carbon-lang/pull/4497 placed
restrictions on a bit, and this is in part flowing back to restrictions
on both NodeId and ImportIRInstId limits.
Thomas Köppe
Jon Ross-Perkins