Treat invalid external function type as a fatal error to prevent the type from being used

- `returnParameterTypesWithoutDynamicTypes()` assumes it won't encounter such types.

test/libsolidity/syntaxTests/iceRegressionTests/calling_external_function_via_local_variable_with_invalid_type.sol

@@ -0,0 +1,8 @@
+contract C {
+    function f() public {
+        function() external returns (function() internal) getCallback;
+        getCallback();
+    }
+}
+// ----
+// TypeError 2582: (76-96): Internal type cannot be used for external function type.

test/libsolidity/syntaxTests/iceRegressionTests/calling_external_function_via_parameter_with_invalid_type.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f(
+        function() external returns (function() internal) getCallback
+    ) public {
+        getCallback();
+    }
+}
+// ----
+// TypeError 2582: (66-86): Internal type cannot be used for external function type.