[clang] [Sema] Add check for bitfield assignments to larger integral types (PR #68276)

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vabridgers wrote:

Hi @AaronBallman , I ran a compile using this change on clang as you asked and have results. The compile ran with no crashes or errors, and produced 1478 bitfield-conversion warnings. I'll show a few examples below. To put that number into context, I enabled -Wconversion to compare the new warning - bitfield-conversion - with the other conversion checks. You can see from the results that while 1478 findings is large, it's less than 10% of the total conversion findings that exist - so looking at those numbers in context is helpful. 

Are there any other suggested improvements for this patch, or do you think it's ok to merge?

![image](https://github.com/llvm/llvm-project/assets/58314289/44313d33-6763-434b-92cb-37df8173209a)




https://github.com/llvm/llvm-project/pull/68276