[clang] [analyzer]:fix valistChecker false negative in windows platform (PR #72951)

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DonatNagyE wrote:

..and it turns out that `ValistChecker` already has a function called `ValistChecker::getVAListAsRegion` that handles different representations of `va_list` under various systems. (I was not familiar with it because it was added by @Xazax-hun after the initial commit https://reviews.llvm.org/D15227 that contains my contributions.)

I'm strongly suspect that extending this function is the "right way" to adapt this checker to the windows environment that you're studying.

https://github.com/llvm/llvm-project/pull/72951