Auto merge of #70164 - eddyb:walk-generic-arg, r=nikomatsakis

ty/walk: iterate `GenericArg`s instead of `Ty`s.

Before this PR, `Ty::walk` only iterated over `Ty`s, but that's becoming an increasing problem with `const` generics, as `ty::Const`s in `Substs` are missed by it.

By working with `GenericArg` instead, we can handle both `Ty`s and `ty::Const`s, but also `ty::Region`s, which used to require ad-hoc mechanisms such as `push_regions`.

I've also removed `TraitRef::input_types`, as it's both long obsolete, and easy to misuse.

src/test/ui/coherence/impl-foreign-for-locally-defined-fundamental.rs

@@ -8,8 +8,8 @@ extern crate coherence_lib as lib;
 use lib::*;
 
 #[fundamental]
-struct Local;
+struct Local<T>(T);
 
-impl Remote for Local {}
+impl Remote for Local<()> {}
 
 fn main() {}