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.

llvm-svn: 286531