Auto merge of #120576 - nnethercote:merge-Diagnostic-DiagnosticBuilder, r=davidtwco

Overhaul `Diagnostic` and `DiagnosticBuilder`

Implements the first part of https://github.com/rust-lang/compiler-team/issues/722, which moves functionality and use away from `Diagnostic`, onto `DiagnosticBuilder`.

Likely follow-ups:
- Move things around, because this PR was written to minimize diff size, so some things end up in sub-optimal places. E.g. `DiagnosticBuilder` has impls in both `diagnostic.rs` and `diagnostic_builder.rs`.
- Rename `Diagnostic` as `DiagInner` and `DiagnosticBuilder` as `Diag`.

r? `@davidtwco`

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -18,8 +18,8 @@ use crate::require_c_abi_if_c_variadic;
 use rustc_ast::TraitObjectSyntax;
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_errors::{
-    codes::*, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
-    FatalError, MultiSpan,
+    codes::*, struct_span_code_err, Applicability, DiagnosticBuilder, ErrorGuaranteed, FatalError,
+    MultiSpan,
 };
 use rustc_hir as hir;
 use rustc_hir::def::{CtorOf, DefKind, Namespace, Res};
@@ -1724,7 +1724,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     pub fn prohibit_generics<'a>(
         &self,
         segments: impl Iterator<Item = &'a hir::PathSegment<'a>> + Clone,
-        extend: impl Fn(&mut Diagnostic),
+        extend: impl Fn(&mut DiagnosticBuilder<'_>),
     ) -> bool {
         let args = segments.clone().flat_map(|segment| segment.args().args);