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_borrowck/src/diagnostics/mutability_errors.rs

@@ -1,5 +1,5 @@
 use hir::ExprKind;
-use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::Node;
@@ -711,7 +711,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 
     fn construct_mut_suggestion_for_local_binding_patterns(
         &self,
-        err: &mut DiagnosticBuilder<'_, ErrorGuaranteed>,
+        err: &mut DiagnosticBuilder<'_>,
         local: Local,
     ) {
         let local_decl = &self.body.local_decls[local];
@@ -1025,7 +1025,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         }
     }
 
-    fn suggest_using_iter_mut(&self, err: &mut DiagnosticBuilder<'_, ErrorGuaranteed>) {
+    fn suggest_using_iter_mut(&self, err: &mut DiagnosticBuilder<'_>) {
         let source = self.body.source;
         let hir = self.infcx.tcx.hir();
         if let InstanceDef::Item(def_id) = source.instance
@@ -1067,12 +1067,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         }
     }
 
-    fn suggest_make_local_mut(
-        &self,
-        err: &mut DiagnosticBuilder<'_, ErrorGuaranteed>,
-        local: Local,
-        name: Symbol,
-    ) {
+    fn suggest_make_local_mut(&self, err: &mut DiagnosticBuilder<'_>, local: Local, name: Symbol) {
         let local_decl = &self.body.local_decls[local];
 
         let (pointer_sigil, pointer_desc) =