[llvm] [BOLT] Refactor BOLT reserved space discovery (PR #90893)

[Amir Ayupov via llvm-commits]

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

Please add to the title that it's NFC or NFCI, or explain in which cases it's supposed to behave differently.

https://github.com/llvm/llvm-project/pull/90893