Allow fallback function to return data.

test/libsolidity/semanticTests/fallback/falback_return.sol

@@ -0,0 +1,18 @@
+contract A {
+    uint public x;
+    fallback () external {
+        if (x == 2) return;
+        x++;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// ()
+// x() -> 1
+// ()
+// x() -> 2
+// ()
+// x() -> 2
+// ()
+// x() -> 2

test/libsolidity/semanticTests/fallback/fallback_argument.sol

@@ -0,0 +1,17 @@
+contract A {
+    uint public x;
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        x = _input.length;
+        return "";
+    }
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// compileViaYul: also
+// EVMVersion: >=byzantium
+// ----
+// f() -> 0x01, 0x40, 0x00
+// x() -> 3

test/libsolidity/semanticTests/fallback/fallback_argument_to_storage.sol

@@ -0,0 +1,16 @@
+contract A {
+    bytes public x;
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        x = _input;
+        return "";
+    }
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// EVMVersion: >=byzantium
+// ----
+// f() -> 0x01, 0x40, 0x00
+// x() -> 0x20, 3, "abc"

test/libsolidity/semanticTests/fallback/fallback_return_data.sol

@@ -0,0 +1,14 @@
+contract A {
+    fallback (bytes calldata _input) external returns (bytes memory) {
+        return _input;
+    }
+    function f() public returns (bool, bytes memory) {
+        (bool success, bytes memory retval) = address(this).call("abc");
+        return (success, retval);
+    }
+}
+// ====
+// compileViaYul: also
+// EVMVersion: >=byzantium
+// ----
+// f() -> 0x01, 0x40, 0x03, 0x6162630000000000000000000000000000000000000000000000000000000000