arrow

GH-40252: [C++] Make span SFINAE standards-conforming to enable compilation with nvcc (#40253)



### Rationale for this change

The current code uses an incomplete type in a SFINAE construct.

Closes #40252 

### What changes are included in this PR?

This PR changes the way that the type is specified to avoid any UB.

### Are these changes tested?

I tested locally that code that originally failed to compile with nvcc now compiles successfully. The same code also compiles under clang and gcc. This is a minimal reproducer:
```
#include <arrow/api.h>
#include <vector>

int main() {
  arrow::util::span<int> x{std::vector<int>{1, 2, 3}};
}
```

I did not include it here because it is a compile-time rather than runtime issue, and because at present it only manifests on nvcc.

### Are there any user-facing changes?

No.

* GitHub Issue: #40252

Authored-by: Vyas Ramasubramani <vyasr@nvidia.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>