Don't require nullability on 'va_list' harder.

Platform headers don't always define 'va_list' in terms of Clang's
'__builtin_va_list', so in addition to checking for our own
synthesized decl, also just look for typedefs literally named
'va_list'. Better to err on the side of false negatives here.

Fix-up for rdar://problem/25846421.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@286531 91177308-0d34-0410-b5e6-96231b3b80d8