[clang] [llvm] [NFC][analyzer] Rename `ValistChecker.cpp` to `VAListChecker.cpp` (PR #157438)

[Donát Nagy via llvm-commits]

NagyDonat wrote:

As this patch is very trivial and I already promised it within https://github.com/llvm/llvm-project/pull/156682 (which was reviewed), I'll merge it without a separate review when the CI succeeds. 

https://github.com/llvm/llvm-project/pull/157438