[libc++] Finish the transition to 0-1 macros

We migrated several macros from `#ifndef` to` #if MACRO` (defining the macro to `0` or `1`) in order to avoid `-Wundef` types of failures. However, we still have several macros that are using `#ifdef`. As a result, we have to check how each macro is used before using it, which is overall more confusing.

We should either finish the transition or at least clearly state which macros will stay `#ifndef` and the rationale for that.