[llvm] [BOLT] Remove mutable from BB::LayoutIndex (PR #93224)

[Amir Ayupov via llvm-commits]

https://github.com/aaupov requested changes to this pull request.

Thank you for working on this. The diff is moving in the right direction, but as evidenced by the amount of code to avoid setting BB->LayoutIndex and associated overheads, it's not a good idea and we should still set it. 

Please define a helper that given a block order sets BB->LayoutIndex, as part of FunctionLayout class.

https://github.com/llvm/llvm-project/pull/93224