[clang] [clang] Better bitfield access units (PR #65742)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I'm planning to take a closer look at this when I have more time.  Sorry I haven't been more responsive here.

One very brief note: in the comments in the code, you might want to distinguish between the semantic width of a bitfield (i.e. the C standard notion of a "memory location", which has ABI significance), vs. the accesses we choose to generate (we don't need to generate no-op reads/writes).

https://github.com/llvm/llvm-project/pull/65742