Re-enable unreachable code checks.

VTK-m is disabling unreachable code warnings on MSVC, since this pattern causes some warnings that we can safely ignore:

struct A
{
  int foo() { return myInt; }
  int myInt;
};

struct B
{
  int foo() { throw ...; }
};

template <typename Fooer>
struct DoFoo
{
  void bar()
  {
    Fooer fooer;
    int foo = fooer.foo();
    std::cerr << foo << "\n"; // unreachable when Fooer is B
  }
};

template class DoFoo<A>; // ok
template class DoFoo<B>; // emits unreachable code warning at indicated line.

This is popping up with read-only storage types, e.g. in StorageImplicit::PrepareForOutput, which just throws.

Rather than disable this useful warning, we should eventually look into other ways to fix the issue.