Fixes invalid function calls to literals inside tuple assignment's LHS.

test/libsolidity/syntaxTests/types/function_call_fail.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f(uint y) public pure {
+        (4(y)) = 2;
+    }
+}
+// ----
+// TypeError: (59-63): Type is not callable
+// TypeError: (59-63): Expression has to be an lvalue.
+// TypeError: (67-68): Type int_const 2 is not implicitly convertible to expected type tuple().

test/libsolidity/syntaxTests/types/function_call_fail2.sol

@@ -0,0 +1,7 @@
+contract C {
+    function f(uint y) public pure returns (uint) {
+        (f(y)) = 2;
+    }
+}
+// ----
+// TypeError: (74-78): Expression has to be an lvalue.