[Clang][Wswitch-default] Warning for enum even completely covered the cases (#75900)

 Adding a test case that this warns even for completely covered switches.

Author: hstk30-hw

clang/test/Sema/switch-default.c

@@ -15,3 +15,14 @@ int f2(int a) {
   }
   return a;
 }
+
+// Warn even completely covered Enum cases(GCC compatibility).
+enum E { A, B };
+enum E check_enum(enum E e) {
+  switch (e) {                // expected-warning {{'switch' missing 'default' label}}
+    case A: break;
+    case B: break;
+  }
+  return e;
+}
+