[AST] Default attributes vector size

Hi.

I'm writing this mail to get some attention to my small contribution.

https://github.com/llvm-mirror/clang/blob/master/include/clang/AST/AttrIterator.h#L41

Obviously, SmallVector was used here to reduce allocations. But it looks like default size is not the best in that case.

Here is my patch to fix that: https://reviews.llvm.org/D23329

Best regards
Eugene