Forbid private library functions to be attached outside their declaration scope

test/libsolidity/semanticTests/using/private_library_function.sol

@@ -0,0 +1,10 @@
+library L {
+    using {L.privateFunction} for uint;
+    function privateFunction(uint x) private pure returns (uint) { return x + 1; }
+    function f() public pure returns (uint) {
+        uint x = 1;
+        return x.privateFunction();
+    }
+}
+// ----
+// f() -> 2