Validate immutable variables

test/libsolidity/syntaxTests/immutable/inheritance_ctor_argument.sol

@@ -0,0 +1,14 @@
+contract B {
+    uint immutable x;
+
+    constructor(uint _x) public {
+        x = _x;
+    }
+}
+
+contract C is B {
+    uint immutable y;
+    constructor() B(y = 3) public { }
+}
+// ----
+// TypeError: (155-156): Immutable variables can only be initialized inline or assigned directly in the constructor.