creating a `TypeErrCtxt` should only be possible if we emit an error

[lcnr]

there are a lot of methods on TypeErrCtxt which should only be used for diagnostics. To assert that this is the case we should emit a delay_span_bug when creating (or probably when dropping so that -Ztreat-err-as-bug keeps working nicely) this context.

This means that we can't use infcx.err_ctxt().check_region_obligations_and_report_errors() as that creates a TypeErrCtxt even though we don't emit any error.

Its probably easiest to use the same split as select_all_or_error, always using resolve_regions which returns a #[must_use] list of errors and then manually calling report_region_errors