Give DiagnosticBuilder a default type.

`IntoDiagnostic` defaults to `ErrorGuaranteed`, because errors are the
most common diagnostic level. It makes sense to do likewise for the
closely-related (and much more widely used) `DiagnosticBuilder` type,
letting us write `DiagnosticBuilder<'a, ErrorGuaranteed>` as just
`DiagnosticBuilder<'a>`. This cuts over 200 lines of code due to many
multi-line things becoming single line things.

compiler/rustc_middle/src/ty/mod.rs

@@ -1481,7 +1481,7 @@ impl<'tcx> OpaqueHiddenType<'tcx> {
         other: &Self,
         opaque_def_id: LocalDefId,
         tcx: TyCtxt<'tcx>,
-    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'tcx> {
         if let Some(diag) = tcx
             .sess
             .dcx()