Account for incorrect where T::Assoc = Ty bound

Provide suggestoin to constrain trait bound for associated type.
Revert incorrect changes to `missing-bounds` test.

Address part of #20041.

compiler/rustc_ast_passes/src/lib.rs

@@ -6,6 +6,7 @@
 
 #![feature(iter_is_partitioned)]
 #![feature(box_patterns)]
+#![feature(let_else)]
 #![recursion_limit = "256"]
 
 pub mod ast_validation;