Test test_async_reduce_allocator_on_then_after in async_reduce.cu has
"KNOWN_FAILURE;" in the middle of a code block.  This results in "statement is
unreachable" compiler errors from NVC++ on the following line.

```
"/proj/cuda/thrust/main/testing/async_reduce.cu", line 978: error: statement is
          unreachable
      ASSERT_EQUAL_QUIET(stream1, f2.stream().native_handle());
      ^
          detected during:
            instantiation of "void unittest::for_each_type<TypeList, Function,
                      T, i>::operator()(U) [with TypeList=NumericTypes,
                      Function=test_async_reduce_allocator_on_then_after,
                      T=char, i=0U, U=size_t]" at line 537 of
                      "/proj/cuda/thrust/main/testing/unittest/testframework.h"
            instantiation of "void VariableUnitTest<TestName, TypeList>::run()
                      [with TestName=test_async_reduce_allocator_on_then_after,
                      TypeList=NumericTypes]"
```

The compiler error is correct.  KNOWN_FAILURE expands to a throw expression,
so everything after it is dead code.

Fix the problem by putting everything after KNOWN_FAILURE in a "#if 0" block.