Disallow external function pointers as immutables.

test/libsolidity/semanticTests/immutable/complexGetter.sol

@@ -1,12 +0,0 @@
-contract C {
-    function() external returns (uint, uint) immutable public x = this.f;
-    function f() external pure returns (uint, uint) {
-        return (1, 2);
-    }
-
-    function test() external returns (uint, uint) {
-        return this.x()();
-    }
-}
-// ----
-// test() -> 1, 2

test/libsolidity/semanticTests/immutable/external_function_pointer.sol

@@ -1,20 +0,0 @@
-contract D {
-	function f() external view returns (uint256) {
-		return 42;
-	}
-}
-contract C {
-	D d;
-	function() external view returns(uint256) immutable z;
-	constructor() public {
-		d = new D();
-		z = d.f;
-	}
-	function f() public view returns (uint256) {
-	assert(z.address == address(d));
-	assert(z.selector == D.f.selector);
-		return z();
-	}
-}
-// ----
-// f() -> 42

test/libsolidity/syntaxTests/immutable/external_function_pointer.sol

@@ -0,0 +1,5 @@
+contract C {
+    function() external immutable f;
+}
+// ----
+// TypeError: (17-48): Immutable variables of external function type are not yet supported.