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).
At present, lower depends on `Check::SemIRDiagnosticConverter` for debug
info. That was to support a quick implementation of debug info, but
isn't great because it's both an unusual dependency on check's
implementation, and relying on diagnostic structures for debug info.
This cleans that up by splitting relevant logic out to a library in
sem_ir, and having lowering use sem_ir's library instead of check's.
Additionally, a small refactoring of `Parse::TreeAndSubtrees` to allow
getting locations in lowering without going through a `DiagnosticLoc`.
I'm adding `Parse::GetTreeAndSubtreesFn` in because it's a complex
signature to have in so many spots.
I chose to have `ResolveNodeId` return a `SmallVector` because it seemed
likely to be fairly compact, but that could also be using an optional
callback to handle resolved node IDs, possibly just returning the last
entry. This could be switched if preferred.
Note this change shouldn't affect behavior, it's just moving code
around.
---------
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>
Jon Ross-Perkins
Boaz Brickner
