[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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@330627 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/experimental/simd/nothing_to_do.pass.cpp

@@ -0,0 +1,12 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include <experimental/simd>
+
+int main() {}