Rename DiagnosticBuilder as Diag.

Much better!

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

compiler/rustc_infer/src/errors/note_and_explain.rs

@@ -1,8 +1,7 @@
 use crate::fluent_generated as fluent;
 use crate::infer::error_reporting::nice_region_error::find_anon_type;
 use rustc_errors::{
-    AddToDiagnostic, DiagnosticBuilder, EmissionGuarantee, IntoDiagnosticArg,
-    SubdiagnosticMessageOp,
+    AddToDiagnostic, Diag, EmissionGuarantee, IntoDiagnosticArg, SubdiagnosticMessageOp,
 };
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_span::{symbol::kw, Span};
@@ -165,7 +164,7 @@ impl RegionExplanation<'_> {
 impl AddToDiagnostic for RegionExplanation<'_> {
     fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
         self,
-        diag: &mut DiagnosticBuilder<'_, G>,
+        diag: &mut Diag<'_, G>,
         f: F,
     ) {
         diag.arg("pref_kind", self.prefix);