[llvm] [nfc][InstrProfReader]Store header fields in native endianness (PR #92947)

[Kazu Hirata via llvm-commits]

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

Thank you so much for cleaning this up!  LGTM modulo one minor comment.

https://github.com/llvm/llvm-project/pull/92947