[Clang] Added testcases for -Wunused-but-set-parameter

Clang now emits warnings for them after D122271 (shared logic with -Wunused-but-set-variable)

clang/test/Sema/warn-unused-but-set-parameters.c

@@ -24,3 +24,11 @@ void f3(struct S s) { // expected-warning{{parameter 's' set but not used}}
   struct S t;
   s = t;
 }
+
+void f4(int j) { // expected-warning{{parameter 'j' set but not used}}
+    j++;
+}
+
+void f5(int k) { // expected-warning{{parameter 'k' set but not used}}
+    --k;
+}