Rename DiagnosticBuilder as Diag.

Much better!

Note that this involves renaming (and updating the value of)
`DIAGNOSTIC_BUILDER` in clippy.

compiler/rustc_trait_selection/src/traits/util.rs

@@ -3,7 +3,7 @@ use std::collections::BTreeMap;
 use super::NormalizeExt;
 use super::{ObligationCause, PredicateObligation, SelectionContext};
 use rustc_data_structures::fx::FxHashSet;
-use rustc_errors::DiagnosticBuilder;
+use rustc_errors::Diag;
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::{InferCtxt, InferOk};
 use rustc_middle::ty::GenericArgsRef;
@@ -46,7 +46,7 @@ impl<'tcx> TraitAliasExpansionInfo<'tcx> {
     /// trait aliases.
     pub fn label_with_exp_info(
         &self,
-        diag: &mut DiagnosticBuilder<'_>,
+        diag: &mut Diag<'_>,
         top_label: &'static str,
         use_desc: &str,
     ) {