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_hir_analysis/src/check/check.rs

@@ -1248,7 +1248,7 @@ fn check_enum(tcx: TyCtxt<'_>, def_id: LocalDefId) {
 fn detect_discriminant_duplicate<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>) {
     // Helper closure to reduce duplicate code. This gets called everytime we detect a duplicate.
     // Here `idx` refers to the order of which the discriminant appears, and its index in `vs`
-    let report = |dis: Discr<'tcx>, idx, err: &mut Diagnostic| {
+    let report = |dis: Discr<'tcx>, idx, err: &mut DiagnosticBuilder<'_>| {
         let var = adt.variant(idx); // HIR for the duplicate discriminant
         let (span, display_discr) = match var.discr {
             ty::VariantDiscr::Explicit(discr_def_id) => {

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -78,7 +78,7 @@ use std::num::NonZero;
 
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_errors::ErrorGuaranteed;
-use rustc_errors::{pluralize, struct_span_code_err, Diagnostic, DiagnosticBuilder};
+use rustc_errors::{pluralize, struct_span_code_err, DiagnosticBuilder};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::Visitor;
 use rustc_index::bit_set::BitSet;