[llvm] [PGO] Add support for writing previous indexed format (PR #84505)

[Teresa Johnson via llvm-commits]

================
@@ -291,6 +291,15 @@ cl::opt<bool> DropProfileSymbolList(
     cl::desc("Drop the profile symbol list when merging AutoFDO profiles "
              "(only meaningful for -sample)"));
 
+// Temporary support for writing the previous version of the format, to enable
+// some forward compaitibility.
----------------
teresajohnson wrote:

done

https://github.com/llvm/llvm-project/pull/84505