[libc++] Fix std::move algorithm with trivial move-only types

As reported in https://reviews.llvm.org/D151953#4472195, the std::move
algorithm (and various other functions that relied on it) stopped working
after starting to use `__constexpr_memmove` in its implementation. This
patch fixes the underlying issue in `__constexpr_memmove` and adds tests
for various related algorithms and functions that were not exercising
trivial move-only types.

Differential Revision: https://reviews.llvm.org/D154613