refactor: extract common pat_is_wild to clippy_utils

This function was previously defined for the iter_kv_map,
for_kw_map, and unused_enumerate_index lints. This commit extracts
it into clippy_utils.

clippy_utils/src/lib.rs

@@ -2960,3 +2960,15 @@ op_utils! {
     Shl    ShlAssign
     Shr    ShrAssign
 }
+
+/// Returns `true` if the pattern is a `PatWild`, or is an ident prefixed with `_`
+/// that is not locally used.
+pub fn pat_is_wild<'tcx>(cx: &LateContext<'tcx>, pat: &'tcx PatKind<'_>, body: impl Visitable<'tcx>) -> bool {
+    match *pat {
+        PatKind::Wild => true,
+        PatKind::Binding(_, id, ident, None) if ident.as_str().starts_with('_') => {
+            !visitors::is_local_used(cx, body, id)
+        },
+        _ => false,
+    }
+}