Validate immutable variables

test/libsolidity/syntaxTests/immutable/ctor_initialization_indirect_reading.sol

@@ -0,0 +1,10 @@
+contract C {
+    uint immutable x;
+    constructor() public {
+        x = f();
+    }
+
+    function f() public pure returns (uint) { return 3 + x; }
+}
+// ----
+// TypeError: (143-144): Immutable variables cannot be read during contract creation time, which means they cannot be read in the constructor or any function or modifier called from it.