[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

include/module.modulemap

@@ -550,6 +550,10 @@ module std [system] {
       header "experimental/regex"
       export *
     }
+    module simd {
+      header "experimental/simd"
+      export *
+    }
     module set {
       header "experimental/set"
       export *