check_attrs: Warn when #[macro_export] is used on macros 2.0

The compiler should emit a more specific error when the `#[macro_export]`
attribute is present on a decl macro, instead of silently ignoring it.

This commit adds the required error message in rustc_passes/messages.ftl,
 as well as a note. A new variant is added to the `errors::MacroExport`
enum, specifically for the case where the attribute is added to a macro
2.0.

compiler/rustc_passes/src/errors.rs

@@ -690,6 +690,10 @@ pub enum MacroExport {
     #[diag(passes_macro_export)]
     Normal,
 
+    #[diag(passes_macro_export_on_decl_macro)]
+    #[note]
+    OnDeclMacro,
+
     #[diag(passes_invalid_macro_export_arguments)]
     UnknownItem { name: Symbol },