[libc++] Refactor __perfect_forward, bind_front and not_fn

This patch fixes the constrains on the __perfect_forward constructor
and its call operators, which were incorrect. In particular, it makes
sure that we closely follow [func.require], which basically says that
we must deliver the bound arguments with the appropriate value category
or make the call ill-formed, but not silently fall back to using a
different value category.

As a fly-by, this patch also:
- Adds types __bind_front_t and __not_fn_t to make the result of
  calling bind_front and not_fn more opaque, and improve diagnostics
  for users.
- Adds a bunch of tests for bind_front and remove some that are now
  redundant.
- Adds some missing _LIBCPP_HIDE_FROM_ABI annotations.

Immense thanks to @tcanens for raising awareness about this issue, and
providing help with the = delete bits.

Differential Revision: https://reviews.llvm.org/D107199