Finishes external call implementation.

test/libsolidity/semanticTests/functionCall/external_call.sol

@@ -0,0 +1,17 @@
+pragma solidity >= 0.6.0;
+
+contract C {
+    function g(uint n) external pure returns (uint) {
+        return n + 1;
+    }
+
+    function f(uint n) public view returns (uint) {
+        return this.g(2 * n);
+    }
+}
+
+// ====
+// compileViaYul: also
+// ----
+// g(uint256): 4 -> 5
+// f(uint256): 2 -> 5