[libcxx] implement <experimental/simd> declarations based on P0214R7.

Summary:
The patch includes all declarations, and also implements the following features:
* ABI.
* narrowing-conversion related SFIANE, including simd<> ctors and (static_)simd_cast.

Reviewers: mclow.lists, EricWF

Subscribers: lichray, sanjoy, MaskRay, cfe-commits

Differential Revision: https://reviews.llvm.org/D41148

llvm-svn: 330627