Adding Stack Height Checker and modifying the number of POP instructions to appropriately provide the pointer address

test/libsolidity/syntaxTests/functionTypes/external_functions_with_variable_number_of_stack_slots.sol

@@ -0,0 +1,8 @@
+contract C {
+    function f (address) external returns (bool) {
+        this.f{gas: 42}.address;
+    }
+}
+// ----
+// Warning 6321: (56-60): Unnamed return variable can remain unassigned. Add an explicit return with value to all non-reverting code paths or name the variable.
+// Warning 2018: (17-102): Function state mutability can be restricted to view