[llvm] [nfc][InstrFDO]Encapsulate header writes in a class member function (PR #90142)

[Mingming Liu via llvm-commits]

minglotus-6 wrote:

thanks for the reviews and suggestions!

I will use 'add suggestions to batch' next time (I forgot this feature is available) :/

https://github.com/llvm/llvm-project/pull/90142