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

[via cfe-commits]

mzyKi 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.

Thanks for your review.I worte ```isWinValistType``` incorrectly.And I will try to find another solution.

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