[libc++][NFC] Add __element_count and use it in the constexpr C functions

This makes it less ambiguous what the parameter is meant to get.

Reviewed By: #libc, ldionne

Spies: ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D152040

GitOrigin-RevId: 355f4667446bf3ef38d3b52485b64d1a71a47507
4 files changed