Remove LintDiagnostic::msg

* instead simply set the primary message inside the lint decorator functions
* it used to be this way before [#]101986 which introduced `msg` to prevent
  good path delayed bugs (which no longer exist) from firing under certain
  circumstances when lints were suppressed / silenced
* this is no longer necessary for various reasons I presume
* it shaves off complexity and makes further changes easier to implement

Author: fmease

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -564,8 +564,8 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
                     lint::builtin::INLINE_NO_SANITIZE,
                     hir_id,
                     no_sanitize_span,
-                    "`no_sanitize` will have no effect after inlining",
                     |lint| {
+                        lint.primary_message("`no_sanitize` will have no effect after inlining");
                         lint.span_note(inline_span, "inlining requested here");
                     },
                 )

compiler/rustc_errors/src/diagnostic.rs

@@ -200,8 +200,6 @@ pub trait SubdiagMessageOp<G: EmissionGuarantee> =
 pub trait LintDiagnostic<'a, G: EmissionGuarantee> {
     /// Decorate and emit a lint.
     fn decorate_lint<'b>(self, diag: &'b mut Diag<'a, G>);
-
-    fn msg(&self) -> DiagMessage;
 }
 
 #[derive(Clone, Debug, Encodable, Decodable)]