Re-commit r330627 "[libcxx] implement <experimental/simd> declarations based on P0214R7."

There are 3 changes:
* Renamed genertor.pass.cpp to generator.pass.cpp
* Removed nothing_to_do.pass.cpp
* Mark GCC 4.9 as UNSUPPORTED for the test files that have negative
  narrowing conversion SFINAE test (see GCC PR63723).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@330655 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/experimental/simd/simd.casts/simd_cast.pass.cpp

@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+//                     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.
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++98, c++03
+
+// <experimental/simd>
+//
+// [simd.casts]
+// template <class T, class U, class Abi> see below simd_cast(const simd<U, Abi>&);
+#include <experimental/simd>
+#include <cstdint>
+
+using namespace std::experimental::parallelism_v2;
+
+static_assert(std::is_same<decltype(simd_cast<int32_t>(native_simd<int32_t>())),
+                           native_simd<int32_t>>::value,
+              "");
+
+static_assert(
+    std::is_same<decltype(simd_cast<int64_t>(fixed_size_simd<int32_t, 4>())),
+                 fixed_size_simd<int64_t, 4>>::value,
+    "");
+
+static_assert(std::is_same<decltype(simd_cast<fixed_size_simd<int64_t, 1>>(
+                               simd<int32_t, simd_abi::scalar>())),
+                           fixed_size_simd<int64_t, 1>>::value,
+              "");
+
+static_assert(std::is_same<decltype(simd_cast<simd<int64_t, simd_abi::scalar>>(
+                               fixed_size_simd<int32_t, 1>())),
+                           simd<int64_t, simd_abi::scalar>>::value,
+              "");
+
+int main() {}