[llvm] Fix compression header size check in ELF writer (PR #66888)

[David Blaikie via llvm-commits]

dwblaikie wrote:

Could this be unit tested, then? (I realize the class is currently in the .cpp file, so would have to be moved into a header so it'd be accessible from a unit test)

https://github.com/llvm/llvm-project/pull/66888