[llvm] [LLVM] Add option to store Parent-pointer in ilist_node_base (PR #94224)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> LGTM assuming this has no undue impact on build times.

The first test I ran reduced had no significant impact on compile times [(results)](http://llvm-compile-time-tracker.com/compare.php?from=b2bd024384b484647da9fd9863bf6f77b5731949&to=f50c98bee78cec619e8e79f6386b70af5994abe2&stat=instructions:u), other than a slight increase in the time to build clang; this increase in time-to-build-clang is more than compensated for by the followup patch for InsertPosition [(results)](http://llvm-compile-time-tracker.com/compare.php?from=f50c98bee78cec619e8e79f6386b70af5994abe2&to=3e5b1785fafacea2d3302d4ca9a4f85cb4cd808c&stat=instructions:u). I'll submit a quick check to the tracker to verify that the recent commits and rebase hasn't affected compile times, but most likely it will be fine. Thank you for the review!

https://github.com/llvm/llvm-project/pull/94224