[libc++] LWG3001: add `remove_extent_t` to `weak_ptr::element_type`.

Also fix a few places in the `shared_ptr` implementation where
`element_type` was passed to the `__is_compatible` helper. This could
result in `remove_extent` being applied twice to the pointer's template
type (first by the definition of `element_type` and then by the helper),
potentially leading to somewhat less readable error messages for some
incorrect code.

Differential Revision: https://reviews.llvm.org/D112092