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

[Nathan Sidwell via cfe-commits]

urnathan wrote:

@rjmccall here's a refactoring along the lines you suggested. Once one stops worrying about rescanning when the next access unit fails to accumulate into the current one, things get simpler. The compiler should be able to turn the boolean conditional flow within the loop body into direct control flow. 

Would you like the API change I introduced in the previous patch (having `accumulateBitFields` return the following non-bitfield) broken out into a separate PR?

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