[libc++] Implement LWG 2911 - add an is_aggregate type-trait

Summary:
This patch implements http://cplusplus.github.io/LWG/lwg-defects.html#2911.

I'm putting this up for review until __is_aggregate is added to clang (See D31513)

Reviewers: mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D31515

llvm-svn: 300126