Optimize Line string method (#4739)

Co-authored-by: cobalt <61329810+cobaltt7@users.noreply.github.com>

Author: av-runner

src/black/lines.py

@@ -484,10 +484,10 @@ def __str__(self) -> str:
         leaves = iter(self.leaves)
         first = next(leaves)
         res = f"{first.prefix}{indent}{first.value}"
-        for leaf in leaves:
-            res += str(leaf)
-        for comment in itertools.chain.from_iterable(self.comments.values()):
-            res += str(comment)
+        res += "".join(str(leaf) for leaf in leaves)
+        comments_iter = itertools.chain.from_iterable(self.comments.values())
+        comments = [str(comment) for comment in comments_iter]
+        res += "".join(comments)
 
         return res + "\n"