Make unused_associated_type_bounds's lint level changeable

compiler/rustc_lint_defs/src/builtin.rs

@@ -3409,6 +3409,7 @@ declare_lint_pass! {
         UNSTABLE_SYNTAX_PRE_EXPANSION,
         UNSUPPORTED_CALLING_CONVENTIONS,
         UNUSED_ASSIGNMENTS,
+        UNUSED_ASSOCIATED_TYPE_BOUNDS,
         UNUSED_ATTRIBUTES,
         UNUSED_CRATE_DEPENDENCIES,
         UNUSED_EXTERN_CRATES,

tests/ui/object-safety/assoc_type_bounds_sized_unnecessary.rs

@@ -11,4 +11,7 @@ fn foo(_: &dyn Foo<Bar = ()>) {}
 //~| WARN: unnecessary associated type bound for not object safe associated type
 //~| WARN: unnecessary associated type bound for not object safe associated type
 
+#[allow(unused_associated_type_bounds)]
+fn bar(_: &dyn Foo<Bar = ()>) {}
+
 fn main() {}