[libc++] No longer support ranges::begin(x) when x is an array of incomplete type.

var-const points out that `ranges::begin` is (non-normatively
but explicitly) always supposed to return a `std::input_or_output_iterator`,
and `Incomplete*` is not a `std::input_or_output_iterator` because it
has no `operator++`. Therefore, we should never return `Incomplete*`
from `ranges::begin(x)`, even when `x` is `Incomplete(&)[]`. Instead,
just SFINAE away.

Differential Revision: https://reviews.llvm.org/D118963