[libc++] Clarify how we pick the typeinfo comparison

This commit makes it clear that the typeinfo comparison implementation
is automatically selected by default, and that the CMake option only
overrides the value. This has been a source of confusion and bugs ever
since we've introduced complexity in that area, so I'm trying to simplify
it while still allowing for some control on the implementation.

Differential Revision: https://reviews.llvm.org/D91574