Remove Clone impl for DiagnosticBuilder.

It seems like a bad idea, asking for diagnostics to be emitted multiple
times.

Author: nnethercote

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -42,7 +42,6 @@ where
 /// access in the methods of `DiagnosticBuilder` here, consider
 /// extending `DiagCtxtFlags`.
 #[must_use]
-#[derive(Clone)]
 pub struct DiagnosticBuilder<'a, G: EmissionGuarantee = ErrorGuaranteed> {
     state: DiagnosticBuilderState<'a>,