Rollup merge of #118495 - weiznich:more_tests_for_on_unimplemented, r=compiler-errors

Restrict what symbols can be used in `#[diagnostic::on_unimplemented]` format strings

This commit restricts what symbols can be used in a format string for any option of the `diagnostic::on_unimplemented` attribute. We previously allowed all the ad-hoc options supported by the internal `#[rustc_on_unimplemented]` attribute. For the stable attribute we only want to support generic parameter names and `{Self}` as parameters.  For any other parameter an warning is emitted and the parameter is replaced by the literal parameter string, so for example `{integer}` turns into `{integer}`. This follows the general design of attributes in the `#[diagnostic]` attribute namespace, that any syntax "error" is treated as warning and subsequently ignored.

r? `@compiler-errors`

Author: TaKO8Ki