Rollup merge of #119897 - compiler-errors:fulfillment-errors, r=lcnr

`OutputTypeParameterMismatch` -> `SignatureMismatch`

I'm probably missing something that made this rename more complicated. What did you end up getting stuck on when renaming this selection error, `@lcnr?`

**also** I renamed the `FulfillmentErrorCode` variants. This is just churn but I wanted to do it forever. I can move it out of this PR if desired.

r? lcnr

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -1291,7 +1291,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                                 }
                                 // The type doesn't implement Clone because of unmet obligations.
                                 for error in errors {
-                                    if let traits::FulfillmentErrorCode::CodeSelectionError(
+                                    if let traits::FulfillmentErrorCode::SelectionError(
                                         traits::SelectionError::Unimplemented,
                                     ) = error.code
                                         && let ty::PredicateKind::Clause(ty::ClauseKind::Trait(