Fix bogus zero-length array test.

Issue 7175 was about .ptr always giving null for a zero-length
static array in a struct, despite the fact that the struct has
size 1 and thus certainly a non-null address.

The test case as modified by 36c9b64, however, checks that a
free-standing zero-length static array on the stack has an
address, which is quite a different scenario. From the spec,
I don't see a reason why zero-sized objects should need any
memory associated with them, so I am reluctant to pessimize
LDC's codegen over this.
Currently spec says nothing for the address of T[0] stack variable. But, with dmd, this test case is guaranteed. So this is a good documentation of dmd backend behavior.
If ldc does not assert this, we can disable this by using `version(LDC)`. That's also good for ldc backend behavior.

If we remove this, we will lost a documentation for dmd. It's not good.