Add helper function to suggest multiple constraints

Add `rustc_middle::ty::suggest_constraining_type_params` that suggests
adding multiple constraints.

`suggest_constraining_type_param` now just forwards params to this new
function.

compiler/rustc_middle/src/lib.rs

@@ -56,6 +56,7 @@
 #![feature(nonzero_ops)]
 #![feature(unwrap_infallible)]
 #![feature(decl_macro)]
+#![feature(drain_filter)]
 #![recursion_limit = "512"]
 #![allow(rustc::potential_query_instability)]