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_middle/src/middle/stability.rs

@@ -9,7 +9,7 @@ use rustc_attr::{
     self as attr, ConstStability, DefaultBodyStability, DeprecatedSince, Deprecation, Stability,
 };
 use rustc_data_structures::unord::UnordMap;
-use rustc_errors::{Applicability, Diagnostic};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_feature::GateIssue;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId, LocalDefIdMap};
@@ -125,7 +125,7 @@ pub fn report_unstable(
 }
 
 pub fn deprecation_suggestion(
-    diag: &mut Diagnostic,
+    diag: &mut DiagnosticBuilder<'_, ()>,
     kind: &str,
     suggestion: Option<Symbol>,
     span: Span,