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

[John McCall via cfe-commits]

rjmccall wrote:

I don't think eliminating a few local variables is an improvement that justifies completely switching algorithms, so I don't think we should take this patch. Reviewing the current algorithm took a lot of work, and reviewing your new algorithm would also take a lot of work, and that's work we shouldn't do for such minor reasons.

If you want to make some code structure improvements to the current algorithm, like extracting code into helper lambdas to reduce nesting/duplication and eliminate the `goto`, that kind of refactor is generally straightforward to review and would be welcome.

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