Rollup merge of #121686 - compiler-errors:rpitit-printing, r=lcnr

Adjust printing for RPITITs

1. Call RPITITs `{synthetic#N}` instead of `{opaque#N}`.
2. Fall back to printing the RPITIT like an opaque even when printed as an `AliasTy`, just like we do for `ty::Alias`.

You could argue that (2.) is misleading, but I believe it's more consistent than naming `{synthetic#N}`, which I assume approximately nobody knows where that def path name comes from.

r? lcnr

Author: GuillaumeGomez