[clang] [CodeGen] Refactor accumulateBitFields into two passes (PR #182814)

[John McCall via cfe-commits]

rjmccall wrote:

Look, you removed some local variables and added some other local variables and a local type. It's not an objective simplification just because you personally understand your new code better. I don't think Clang should take this patch, and I am doing you the courtesy of explaining that rather than leaving you hanging. Of course, Eli may think otherwise, and if he wants to review it for you, I won't object.

https://github.com/llvm/llvm-project/pull/182814