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

[Bill Wendling via cfe-commits]

bwendling wrote:

It shouldn't be necessary to analyze `uint64_t Storage` directly through a debugger. It's handled via the `Bitfield` getters and setters. Is that not sufficient for debugging purposes?

My sinister reason for wanting to do this is that many of the `const_cast<IdentifierInfo>(...)` constructs are there because some of these fields are updated. I'd like to get rid of those by making the storage `mutable` (pardon my French).

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