ast_lowering: Stop wrapping ident matchers into groups

The lowered forms goes to metadata, for example during encoding of macro definitions

compiler/rustc_session/src/lib.rs

@@ -1,5 +1,6 @@
 #![feature(crate_visibility_modifier)]
 #![feature(derive_default_enum)]
+#![feature(if_let_guard)]
 #![feature(let_chains)]
 #![feature(let_else)]
 #![feature(min_specialization)]