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

[Amir Ayupov via llvm-commits]

https://github.com/aaupov approved this pull request.

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Minor nit: I'd also replace "BB:LayoutIndex" with "BB::LayoutIndex" as more idiomatic version.

Otherwise looks good to me, thank you for working on it! Let's wait for internal testing to finish before landing.

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