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

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 *