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/libcxx/double_include.sh.cpp

@@ -151,6 +151,7 @@
 #include <experimental/memory_resource>
 #include <experimental/propagate_const>
 #include <experimental/regex>
+#include <experimental/simd>
 #include <experimental/set>
 #include <experimental/string>
 #include <experimental/type_traits>