Port #[must_use] to new attribute parsing infrastructure

Signed-off-by: Jonathan Brouwer <jonathantbrouwer@gmail.com>

compiler/rustc_attr_parsing/src/attributes/mod.rs

@@ -33,6 +33,7 @@ pub(crate) mod confusables;
 pub(crate) mod deprecation;
 pub(crate) mod inline;
 pub(crate) mod lint_helpers;
+pub(crate) mod must_use;
 pub(crate) mod repr;
 pub(crate) mod semantics;
 pub(crate) mod stability;