[clang] [clang][analyzer] Fix the false positive ArgInitializedness warning on unnamed bit-field (PR #145066)

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Tedlion wrote:

@steakhal I've reverted my change in RegionStore.cpp, since now I realize it is more complicated than I thought. Considring the costum copy construction and direct memory operations(such as the memset), simply considering the unnamed bit-field is an UndefinedVal may be wrong , even though the UndefinedVal in my case. Thank you a lot for discussion. I may work on this afterwards, but leaving the RegionStore unchanged maybe the best choice now.

https://github.com/llvm/llvm-project/pull/145066