[libc++] Define ostream nullptr inserter for >= C++17 only

The `ostream` `nullptr` inserter implemented in 3c125fe8 is missing a C++ version
guard. Normally, `libc++` takes the stance of backporting LWG issues to older
standards modes as was done in 3c125fe8. However, backporting to older standards
modes breaks existing code in popular libraries such as `Boost.Test` and
`Google Test` who define their own overload for `nullptr_t`.

Instead, only apply this `operator<<` overload in C++17 or later.

Fixes https://github.com/llvm/llvm-project/issues/55861.

Differential Revision: https://reviews.llvm.org/D127033