Add helper methods checking for "#[non_exhaustive] that's active"

A check for `#[non_exhaustive]` is often done in combination with
checking whether the type is local to the crate, in a variety of ways.
Create a helper method and standardize on it as the way to check for
this.

clippy_lints/src/unneeded_struct_pattern.rs

@@ -51,7 +51,7 @@ impl LateLintPass<'_> for UnneededStructPattern {
             let variant = cx.tcx.adt_def(enum_did).variant_with_id(did);
 
             let has_only_fields_brackets = variant.ctor.is_some() && variant.fields.is_empty();
-            let non_exhaustive_activated = !variant.def_id.is_local() && variant.is_field_list_non_exhaustive();
+            let non_exhaustive_activated = variant.field_list_has_applicable_non_exhaustive();
             if !has_only_fields_brackets || non_exhaustive_activated {
                 return;
             }