[pstl] Added missing double-underscore prefixes to some types

Summary:
Some of the `move_` types were used with leading double-underscores, but
they were declared without `__` prefix.

This patch adds double-underscores to all `move_` types, and their uses.

Reviewers: MikeDvorskiy, ldionne, EricWF, rodgert, #libc

Reviewed By: MikeDvorskiy, ldionne, #libc

Subscribers: dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D78040