[migrator] Add a viewer utility for child indices to facilitate access. NFC (#8925)

Author: nkcsgexi

include/swift/IDE/APIDigesterData.h

@@ -69,6 +69,9 @@ struct CommonDiffItem: public APIDiffItem {
   SDKNodeKind NodeKind;
   NodeAnnotation DiffKind;
   StringRef ChildIndex;
+private:
+  llvm::SmallVector<uint8_t, 4> ChildIndexPieces;
+public:
   StringRef LeftUsr;
   StringRef RightUsr;
   StringRef LeftComment;
@@ -80,6 +83,7 @@ struct CommonDiffItem: public APIDiffItem {
                  StringRef LeftComment, StringRef RightComment,
                  StringRef ModuleName);
 
+  ArrayRef<uint8_t> getChildIndices() { return ChildIndexPieces; }
   static StringRef head();
   bool operator<(CommonDiffItem Other) const;
   static bool classof(const APIDiffItem *D);

lib/IDE/APIDigesterData.cpp

@@ -59,6 +59,11 @@ CommonDiffItem(SDKNodeKind NodeKind, NodeAnnotation DiffKind,
                  RightUsr(RightUsr), LeftComment(LeftComment),
                  RightComment(RightComment), ModuleName(ModuleName) {
   assert(!ChildIndex.empty() && "Child index is empty.");
+  llvm::SmallVector<StringRef, 4> Pieces;
+  ChildIndex.split(Pieces, ":");
+  std::transform(Pieces.begin(), Pieces.end(),
+                 std::back_inserter(ChildIndexPieces),
+                 [](StringRef Piece) { return std::stoi(Piece); });
 }
 
 StringRef swift::ide::api::CommonDiffItem::head() {