Rename DiagnosticBuilder as Diag.

Much better!

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

compiler/rustc_lint_defs/src/lib.rs

@@ -683,7 +683,7 @@ pub struct BufferedEarlyLint {
     /// `rustc_lint::early::EarlyContextAndPass::check_id`.
     pub lint_id: LintId,
 
-    /// Customization of the `DiagnosticBuilder<'_>` for the lint.
+    /// Customization of the `Diag<'_>` for the lint.
     pub diagnostic: BuiltinLintDiagnostics,
 }