Fix checks for "using for ... global" for libraries.

test/libsolidity/semanticTests/using/library_on_interface.sol

@@ -0,0 +1,18 @@
+using L for I;
+interface I { function f() external pure returns (uint); }
+library L {
+    function execute(I i) internal pure returns (uint) {
+        return i.f();
+    }
+}
+contract C is I {
+    function x() public view returns (uint) {
+        I i = this;
+        return i.execute();
+    }
+    function f() public pure returns (uint) { return 7; }
+}
+// ====
+// compileViaYul: also
+// ----
+// x() -> 7