Update region inference for traits so that a method with
explicit self doesn't incorrectly cause the entire trait to
be tagged as being region-parameterized.

Fixes #5224.
auto merge of #5225 : nikomatsakis/rust/issue-5224, r=nikomatsakis

Update region inference for traits so that a method with explicit self doesn't incorrectly cause the entire trait to be tagged as being region-parameterized.

Fixes #5224.

r? @pcwalton