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

compiler/rustc_span/src/symbol.rs

@@ -1690,6 +1690,7 @@ symbols! {
         suggestion,
         sym,
         sync,
+        synthetic,
         t32,
         target,
         target_abi,