Add fields for extern to EntityWithParamsBase (#4206)

This adds fields to `EntityWithParamsBase` to reflect the intention with
`extern library` design. I'm renaming `decl_id` because it shouldn't be
expected to be assigned anymore. import_ref.cpp I'm deliberately keeping
on `first_owning_decl_id` (which will break when importing `extern
library` declarations). Most other cases are for diagnostics, and I'm
using `latest_decl_id` to try and get the closest declaration to the
error. Note I'm partly splitting out this PR to show the test effect,
which apparently we don't test related cases.

toolchain/check/impl.cpp

@@ -25,7 +25,8 @@ static auto NoteAssociatedFunction(Context& context,
   CARBON_DIAGNOSTIC(ImplAssociatedFunctionHere, Note,
                     "Associated function {0} declared here.", SemIR::NameId);
   const auto& function = context.functions().Get(function_id);
-  builder.Note(function.decl_id, ImplAssociatedFunctionHere, function.name_id);
+  builder.Note(function.latest_decl_id(), ImplAssociatedFunctionHere,
+               function.name_id);
 }
 
 // Gets the self specific of a generic declaration that is an interface member,