Prefer DiagnosticBuilder over Diagnostic in diagnostic modifiers.

There are lots of functions that modify a diagnostic. This can be via a
`&mut Diagnostic` or a `&mut DiagnosticBuilder`, because the latter type
wraps the former and impls `DerefMut`.

This commit converts all the `&mut Diagnostic` occurrences to `&mut
DiagnosticBuilder`. This is a step towards greatly simplifying
`Diagnostic`. Some of the relevant function are made generic, because
they deal with both errors and warnings. No function bodies are changed,
because all the modifier methods are available on both `Diagnostic` and
`DiagnosticBuilder`.

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -18,7 +18,7 @@ use crate::traits::{
     Obligation, ObligationCause, PredicateObligation, PredicateObligations, SelectionContext,
 };
 use rustc_data_structures::fx::FxIndexSet;
-use rustc_errors::Diagnostic;
+use rustc_errors::{DiagnosticBuilder, EmissionGuarantee};
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_infer::infer::{DefineOpaqueTypes, InferCtxt, TyCtxtInferExt};
@@ -58,7 +58,7 @@ pub struct OverlapResult<'tcx> {
     pub involves_placeholder: bool,
 }
 
-pub fn add_placeholder_note(err: &mut Diagnostic) {
+pub fn add_placeholder_note<G: EmissionGuarantee>(err: &mut DiagnosticBuilder<'_, G>) {
     err.note(
         "this behavior recently changed as a result of a bug fix; \
          see rust-lang/rust#56105 for details",