Remove ImportRefUsed (#3934)

This was to track use of a declaration after import, prior to a
redeclaration. Per [discussion on
Discord](https://discord.com/channels/655572317891461132/1217182321933815820/1236016521059237962),
we likely don't need this check due to the change in behavior of
`extern`.

Rather than potentially getting one of many `extern` decls and depending
on it by accident, it is now planned to be _required_ to be imported,
and the library doing a non-`extern` decl must _know_ it's importing the
`extern` decl. The stricter requirement on the library means it now
seems more reasonable to use the `extern` decl.

So kind of rolling back #3831, though keeping `ImportIRInstId` (at least
for now) and keeping `Loaded`/`Unloaded` terminology (seems a nicer
fit).

toolchain/check/handle_function.cpp

@@ -93,7 +93,7 @@ static auto TryMergeRedecl(Context& context, Parse::AnyFunctionDeclId node_id,
     return;
   }
 
-  auto prev_inst_for_merge = ResolvePrevInstForMerge(context, node_id, prev_id);
+  auto prev_inst_for_merge = ResolvePrevInstForMerge(context, prev_id);
   auto prev_function_id =
       GetRedeclFunctionId(context, prev_inst_for_merge.inst);
   if (!prev_function_id.is_valid()) {