When we ship LLVM 16, <ranges> won't be considered experimental anymore.
We might as well do this sooner rather than later.
Differential Revision: https://reviews.llvm.org/D132151
[libc++] add global variable template std::views::empty
Note it is neither a range adaptor, nor a CPO. It is simplify a global variable template.
Reviewed By: #libc, Mordante
Differential Revision: https://reviews.llvm.org/D122996