Readd the matches_macro diag item

This is now used by Clippy

compiler/rustc_span/src/symbol.rs

@@ -893,6 +893,7 @@
         masked,
         match_beginning_vert,
         match_default_bindings,
+        matches_macro,
         maxnumf32,
         maxnumf64,
         may_dangle,

library/core/src/macros/mod.rs

@@ -338,6 +338,7 @@ macro_rules! debug_assert_ne {
 /// ```
 #[macro_export]
 #[stable(feature = "matches_macro", since = "1.42.0")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "matches_macro")]
 macro_rules! matches {
     ($expression:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => {
         match $expression {