Use str_enc_copy_direct to improve performance

str_enc_copy_direct copies the string encoding over without checking the
frozen status of the string. Because we know that we're safe here (we
only use this function when interpolating strings on the stack via a
concatstrings instruction) we can safely skip this check

string.c

@@ -3352,7 +3352,7 @@ rb_str_concat_literals(size_t num, const VALUE *strary)
 
     for (i = 0; i < num; ++i) { len += RSTRING_LEN(strary[i]); }
     str = rb_str_buf_new(len);
-    str_enc_copy(str, strary[0]);
+    str_enc_copy_direct(str, strary[0]);
 
     for (i = s; i < num; ++i) {
         const VALUE v = strary[i];