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/lower/file_context.cpp

@@ -733,7 +733,7 @@ auto FileContext::BuildVtable(const SemIR::Class& class_info)
 
   auto first_owning_decl_loc =
       sem_ir().insts().GetLocId(class_info.first_owning_decl_id);
-  if (first_owning_decl_loc.is_import_ir_inst_id()) {
+  if (first_owning_decl_loc.kind() == SemIR::LocId::Kind::ImportIRInstId) {
     // Emit a declaration of an imported vtable using a(n opaque) pointer type.
     // This doesn't have to match the definition that appears elsewhere, it'll
     // still get merged correctly.