Disable VLA diagnostic in C++1y mode, and add some tests.

Still to do here:
  - we have a collection of syntactic accepts-invalids to diagnose
  - support non-PODs in VLAs, including dynamic initialization /
    destruction
  - runtime checks (and throw std::bad_array_length) for bad bound
  - support VLA capture by reference in lambdas
  - properly support VLAs in range-based for (don't recompute bound)