Handle import refs in TryGetExistingDecl (#7620)

This solves the problem where `interface` imports are incorrectly
diagnosed as duplicate names in impl files.
Follows the implementation logic used in `handle_class.cpp`.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/handle_interface.cpp

@@ -78,9 +78,6 @@ static auto BuildInterfaceDecl(Context& context,
     auto existing_interface_decl = existing_decl->As<SemIR::InterfaceDecl>();
     interface_decl.interface_id = existing_interface_decl.interface_id;
     interface_decl.type_id = existing_interface_decl.type_id;
-    // TODO: If the new declaration is a definition, keep its parameter
-    // and implicit parameter lists rather than the ones from the
-    // previous declaration.
 
     auto prev_decl_generic_id =
         context.interfaces().Get(interface_decl.interface_id).generic_id;