[libc++] Remove a bunch of now unnecessary indirections in __tree (llvm#142397)

Most notably, this removes the notion of a distinct `value_type` and
`__container_value_type` from `__tree`, since these are now always the
same type. There are a few places we need to keep `__value_type` around,
since they are ABI visibile. In these cases `_Tp` is used directly. The
second simplification here is that we use `const value_type&` instead of
`const key_type&` in a few places and make use of the fact that the
comparator is capable of comparing any combination of `key_type` and
`value_type`.

This is a follow-up to llvm#134819.

Author: philnik777