Summary:
[LWG 2911](http://cplusplus.github.io/LWG/lwg-defects.html#2911) adds `std::is_aggregate` to the library, which requires a new builtin trait. This patch implements `__is_aggregate`.


Reviewers: rsmith, majnemer, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: STL_MSFT, cfe-commits

Differential Revision: https://reviews.llvm.org/D31513

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300116 91177308-0d34-0410-b5e6-96231b3b80d8