This PR adds support for loading bit-field members from structs using
the `get_bitfield` operation.
It enables retrieving the address of the bitfield-packed member but does
**not** yet support volatile bitfields this will be addressed in a
future PR.
- Generalizes CIRFPTypeInterface files to CIRTypeInterfaces for future type interfaces additions.
- Renames CIRFPTypeInterface to FPTypeInterface.
- Fixes FPTypeInterface tablegen prefix.
This mirrors incubator changes from https://github.com/llvm/clangir/pull/1713
