Don't require nullability on 'va_list'.

There are many non-portable typedefs, but va_list is one that nobody
ever thinks of as a pointer or an array. (When's the last time you saw
someone check for a NULL va_list?) Make an exception for this one
special type.

Part of rdar://problem/25846421.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@286522 91177308-0d34-0410-b5e6-96231b3b80d8