Fix abi.decode returning single value.

test/libsolidity/syntaxTests/specialFunctions/abidecode/abi_decode_empty.sol

@@ -0,0 +1,7 @@
+contract C {
+    function f() public pure {
+        abi.decode("abc", ());
+    }
+}
+// ----
+// Warning: (52-73): Statement has no effect.

test/libsolidity/syntaxTests/specialFunctions/abidecode/abi_decode_single_return.sol

@@ -0,0 +1,5 @@
+contract C {
+    function f() public pure returns (bool) {
+        return abi.decode("abc", (uint)) == 2;
+    }
+}