[Sol->Yul] Fixing string calldata to bytes calldata conversion.

test/libsolidity/semanticTests/array/copying/bytes_calldata_to_string_calldata.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f(bytes calldata c) public returns (string calldata s) {
+        return string(c);
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f(bytes): 0x20, 3, "abc" -> 0x20, 3, "abc"

test/libsolidity/semanticTests/array/copying/string_calldata_to_bytes_calldata.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f(string calldata s) public returns (bytes calldata m) {
+        return bytes(s);
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f(string): 0x20, 3, "abc" -> 0x20, 3, "abc"