Add bounds checking to unique_ptr<T[]> and shared_ptr<T[]>

[ldionne]

The array specialization of unique_ptr and the base specialization of shared_ptr have operator[]. There should be an ABI configuration that makes it possible to do bounds checking for these types.

[davidben]

Related: #73452

In Chromium, we've just introduced a HeapArray<T> type to be what unique_ptr<T[]> should have been. Even if libc++ colluded with the compiler's array cookie ABI to extract the size, it's still pretty inconvenient for the programmer. You can't take a unique_ptr<T[]> and make a span<T> out of it because the container has lost track of the size.

The other tricky bit is that the Itanium C++ ABI omits array cookies when your type has a trivial destructor. So we'd need a compiler-level ABI break to add the array cookie everywhere.
https://itanium-cxx-abi.github.io/cxx-abi/abi.html#array-cookies

That said, whether we add the array cookie to trivial types, non-trivial types do reliably have an array cookie. And, from what I can tell, the array cookie business is implemented by the compiler, not operator new[]. So even if the application totally customized operator new[], I think libc++ could, with knowledge of the ABI, extract out the length for non-trivial types and harden operator[] without much fuss. But you would need to reliably detect the array ABI.