[clang] [analyzer] Improve messaging in security.VAList (PR #157846)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

I abandoned my plans to implement coverage for the SEI-CERT rule MSC39-C because it turns out that `security.VAList` doesn't model `va_list`s properly if they are passed to functions by value (it associates the metadata with regions) and fixing this system would be too difficult compared to the limited benefits.

https://github.com/llvm/llvm-project/pull/157846