[clang] [CIR] Add support for retrieving bitfield member (PR #145971)

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Andres-Salamanca wrote:

I initially split this out because I also need to introduce a BitfieldInfoAttr, and combining everything in one patch would make it quite long. That said, I was already working on this in another branch, so I’ll go ahead and bring that code over and include get_bitfield here as well.

https://github.com/llvm/llvm-project/pull/145971