[libc++] Rewrite std::to_address to avoid relying on element_type

This is a rough reapplication of the change that fixed std::to_address
to avoid relying on element_type (da456167). It is somewhat different
because the fix to avoid breaking Clang (which caused it to be reverted
in 347f69c5) was a bit more involved.

Differential Revision: https://reviews.llvm.org/D101638