[NFC] update comments from an earlier version of SuffixTree (#89800)

LeafChildren is used in an earlier version of the SuffixTree
implementation to keep track of each nodes' leaf nodes. In the
new/current version, this variable is no longer used, but a comment is
left behind. This patch updates the comment.

Author: xuanzhang816

llvm/lib/Support/SuffixTree.cpp

@@ -242,8 +242,8 @@ void SuffixTree::RepeatedSubstringIterator::advance() {
     unsigned Length = Curr->getConcatLen();
 
     // Iterate over each child, saving internal nodes for visiting, and
-    // leaf nodes in LeafChildren. Internal nodes represent individual
-    // strings, which may repeat.
+    // leaf nodes' SuffixIdx in RepeatedSubstringStarts. Internal nodes
+    // represent individual strings, which may repeat.
     for (auto &ChildPair : Curr->Children) {
       // Save all of this node's children for processing.
       if (auto *InternalChild =