Add missing #[rustc_lint_diagnostics] attributes.

Prior to the previous commit, `#[rust_lint_diagnostics]` attributes
could only be used on methods with an `impl Into<{D,Subd}iagMessage>`
parameter. But there are many other nearby diagnostic methods (e.g.
`Diag::span`) that don't take such a parameter and should have the
attribute.

This commit adds the missing attribute to these `Diag` methods. This
requires adding some missing
`#[allow(rustc::diagnostic_outside_of_impl)]` markers at call sites to
these methods.

compiler/rustc_session/src/parse.rs

@@ -168,6 +168,7 @@ pub fn add_feature_diagnostics<G: EmissionGuarantee>(
 /// This variant allows you to control whether it is a library or language feature.
 /// Almost always, you want to use this for a language feature. If so, prefer
 /// `add_feature_diagnostics`.
+#[allow(rustc::diagnostic_outside_of_impl)] // FIXME
 pub fn add_feature_diagnostics_for_issue<G: EmissionGuarantee>(
     err: &mut Diag<'_, G>,
     sess: &Session,

compiler/rustc_session/src/session.rs

@@ -312,6 +312,7 @@ impl Session {
     ) -> Diag<'a> {
         let mut err = self.dcx().create_err(err);
         if err.code.is_none() {
+            #[allow(rustc::diagnostic_outside_of_impl)]
             err.code(E0658);
         }
         add_feature_diagnostics(&mut err, self, feature);