Validate immutable variables

test/libsolidity/syntaxTests/immutable/ctor_modifier_args.sol

@@ -0,0 +1,12 @@
+contract C {
+    uint immutable x;
+    constructor() readX(x = 3) public { }
+
+    modifier readX(uint _x) {
+        _; f(_x);
+    }
+
+    function f(uint a) internal pure {}
+}
+// ----
+// TypeError: (59-60): Immutable variables can only be initialized inline or assigned directly in the constructor.