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_hir_analysis/src/astconv/mod.rs

@@ -2686,7 +2686,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         &self,
         constrained_regions: FxHashSet<ty::BoundRegionKind>,
         referenced_regions: FxHashSet<ty::BoundRegionKind>,
-        generate_err: impl Fn(&str) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>,
+        generate_err: impl Fn(&str) -> DiagnosticBuilder<'tcx>,
     ) {
         for br in referenced_regions.difference(&constrained_regions) {
             let br_name = match *br {