Refactor `FnCtxt::autoderef` to use an external iterator and to not
register any obligation from the main autoderef loop, but rather to
register them after (and if) the loop successfully completes.
Fixes#24819Fixes#25801Fixes#27631Fixes#31258Fixes#31964Fixes#32320Fixes#33515Fixes#33755
Original issue: https://github.com/rust-lang/rust/issues/21195
Relevant PR: https://github.com/rust-lang/rust/pull/30778
Prior to this commit, if a compiletest testcase included the text
"HELP:" or "NOTE:" (note the colons), then it would indicate to the
compiletest suite that we should verify "help" and "note" expected
messages.
This commit updates this check to also check "HELP" and "NOTE" (not the
absense of colons) so that we always verify "help" and "note" expected
messages.