Address the LocIdAndInst::ReusingLoc TODO (#4211)

The TODO for switching to ReusingLoc (previously Untyped) had been there
for a while, so I'm trying to address it here. The intent had been to be
clearer about when the construction is validated, particularly so that
we aren't accidentally accepting an incorrect NodeId. Note, this does
fix an incorrect use of InvalidNodeId where NoLoc should've been called.

Since this changes the semantics of when `Parse::NodeId` is helpful in
`typed_nodes.h`, I'm doing a pass to either refine or switch to
`Parse::InvalidNodeId` where it compiles. I think most remaining
`Parse::NodeId` examples are things we _should_ be able to refine with a
little more work (versus before where `Parse::NodeId` also indicated
`LocId` construction might be used).

I'm also changing context.h to use `requires` that match what
`LocIdAndInst` has, I think it makes the diagnostics a little better.
And note I do add an overload for `ImportIRInstId`, also matching
`LocIdAndInst`, and widely used for import refs.

toolchain/check/interface.cpp

@@ -34,7 +34,7 @@ auto BuildAssociatedEntity(Context& context, SemIR::InterfaceId interface_id,
   // not the declaration itself.
   auto type_id = context.GetAssociatedEntityType(
       self_type_id, context.insts().Get(decl_id).type_id());
-  return context.AddInst<SemIR::AssociatedEntity>(
+  return context.AddInstReusingLoc<SemIR::AssociatedEntity>(
       context.insts().GetLocId(decl_id),
       {.type_id = type_id, .index = index, .decl_id = decl_id});
 }