Rollup merge of #97478 - JohnTitor:fixme-fn-decl, r=compiler-errors

Remove FIXME on `ExtCtxt::fn_decl()`

`ExtCtxt::fn_decl()` is used like `self.fn_decl(..)` or `self.cx.fn_decl(..)`, coverting it to an assoc fn, for example, makes it inconvenience (e.g. `self.cx.fn_decl(..)` would be longer to represent). Given that, it doesn't seem a "FIXME" thing and unused `self` is okay, I think.

compiler/rustc_expand/src/build.rs

@@ -514,7 +514,7 @@ impl<'a> ExtCtxt<'a> {
         }
     }
 
-    // FIXME: unused `self`
+    // `self` is unused but keep it as method for the convenience use.
     pub fn fn_decl(&self, inputs: Vec<ast::Param>, output: ast::FnRetTy) -> P<ast::FnDecl> {
         P(ast::FnDecl { inputs, output })
     }