[clang] [Clang] replace 'bitfield' with 'bit-field' for consistency (PR #117881)

[Aaron Ballman via cfe-commits]

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

LGTM assuming precommit CI comes back green. No need for a release note as this is just fixing a typo, really.

https://github.com/llvm/llvm-project/pull/117881