Rollup merge of #123395 - compiler-errors:postfix-matches-fixes-2, r=petrochenkov

More postfix match fixes

These affect diagnostics only, as far as I can tell. I'm too lazy to come up with UI tests, but I could be convinced otherwise.

Specifically, I think changing the precedence computation actually doesn't change anything, but tweaking `contains_exterior_struct_lit` does mean that some diagnostics will begin parenthesizing `S {}.match {}`.

Author: matthiaskrgr