zoecarver 3ed89b51da [Take 2] [libc++] Make rotate a constexpr.
This patch makes `std::rotate` a constexpr. In doing so, this patch also
updates the internal `__move` and `__move_backward` funtions to be
constexpr.

This patch was previously reverted in ed653184ac63 because it was missing
some UNSUPPORTED markup for older compilers. This commit adds it.

Differential Revision: https://reviews.llvm.org/D65721
2020-09-14 18:14:46 -04:00
..