[libc++] Implement P1007R3: std::assume_aligned

This supersedes and incoroporates content from both D108906 and D54966,
and also some original content.

Co-Authored-by: Marshall Clow <mclow.lists@gmail.com>
Co-Authored-by: Gonzalo Brito Gadeschi

Differential Revision: https://reviews.llvm.org/D118938