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

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vabridgers wrote:

Hi @AaronBallman and @erichkeane , any improvements you could suggest? I could expand the testing for sure. Before I invest time in that, I'd like to be sure the general approach is ok in Sema. Thanks.

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