Reformulate `point_at_expr_source_of_inferred_type` to be more accurate
Be more accurate when deducing where along the several usages of a binding it is constrained to be some type that is incompatible with an expectation.
This also renames the method to `note_source_of_type_mismatch_constraint` because I prefer that name, though I guess I can revert that. (Also drive-by rename `note_result_coercion` -> `suggest_coercing_result_via_try_operator`, because it's suggesting, not noting!)
This PR is (probably?) best reviewed per commit, but it does regress a bit only to fix it later on, so it could also be reviewed as a whole if that makes the final results more clear.
r? `@estebank`
7bfccb3d7f