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).

llvm-svn: 330655