Refactor LocId to merge in SemIRLoc (#5284)

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).

toolchain/check/check_unit.cpp

@@ -407,7 +407,8 @@ auto CheckUnit::CheckRequiredDeclarations() -> void {
         function.extern_library_id == context_.sem_ir().library_id()) {
       auto function_loc_id =
           context_.insts().GetLocId(function.non_owning_decl_id);
-      CARBON_CHECK(function_loc_id.is_import_ir_inst_id());
+      CARBON_CHECK(function_loc_id.kind() ==
+                   SemIR::LocId::Kind::ImportIRInstId);
       auto import_ir_id = context_.sem_ir()
                               .import_ir_insts()
                               .Get(function_loc_id.import_ir_inst_id())