[clang] [CIR] Record how far a bit-field's declared types reach on x86_64 (PR #220069)

[Andy Kaylor via cfe-commits]

https://github.com/andykaylor commented:

This feels like it's evolving without a plan. This is at least the second time we've stuck a zero-length array into a record as a hint to the ABI. That seems very much like a hack to make it work. Could we instead somehow make this part of the field description for the bitfield?

https://github.com/llvm/llvm-project/pull/220069