[clang] [NFC][IdentifierInfo] Use llvm::Bitfield for the bitfield (PR #79366)

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cor3ntin wrote:

Thanks for this patch!

Everything that's touching identifiers is going to be on the hot path, so we want to make sure this does not introduce compile times performance regression with this change.

A lot of work was done recently to ensure the debugging experience of these bitfield, so we should make sure that does not regress either.

I think the readability improvements here are marginal, but I'd like opinion from other folks.

https://github.com/llvm/llvm-project/pull/79366