Implement virtual keyword

test/libsolidity/semanticTests/functionCall/base_base_overload.sol

@@ -1,11 +1,11 @@
 contract BaseBase {
 	uint public x;
 	uint public y;
-	function init(uint a, uint b) public {
+	function init(uint a, uint b) public virtual {
 		x = b;
 		y = a;
 	}
-	function init(uint a) public {
+	function init(uint a) public virtual {
 		x = a + 1;
 	}
 }

test/libsolidity/semanticTests/viaYul/virtual_functions.sol

@@ -2,7 +2,7 @@ contract X {
     function f() public returns (uint x) {
         x = g();
     }
-    function g() public returns (uint x) {
+    function g() public virtual returns (uint x) {
         x = 2;
     }
 }