Auto merge of #125413 - lcnr:ambig-drop-region-constraints, r=compiler-errors

drop region constraints for ambiguous goals

See the comment in `compute_external_query_constraints`. While the underlying issue is preexisting, this fixes a bug introduced by #125343.

It slightly weakens the leak chec, even if we didn't have any test which was affected. I want to write such a test before merging this PR.

r? `@compiler-errors`

Author: bors