[libc++] Simplify the tuple constructor overload set

This uses conditional explicit to avoid having two overloads for implicit/explicit conversions.

Reviewed By: ldionne, #libc

Spies: jrtc27, dblaikie, #clang-vendors, #libc_vendors, aaron.ballman, libcxx-commits

Differential Revision: https://reviews.llvm.org/D148432

Author: philnik777