Tweak region inference to ignore constraints like 'a <= 'static, since they
have no value. This also ensures that we can handle some obscure cases of fn
subtyping with bound regions that we didn't used to handle correctly.

Fixes #13974.
auto merge of #13975 : nikomatsakis/rust/issue-13794-fn-subtyping-and-static, r=pnkfelix

Tweak region inference to ignore constraints like `'a <= 'static`, since they
have no value. This also ensures that we can handle some obscure cases of fn
subtyping with bound regions that we didn't used to handle correctly.

Fixes #13974.