[libc++] Only make comparators transparent in __tree if they don't cause a conversion (#179453)

We're currently unwrapping `less<T>` even if the `key_type` isn't `T`.
This causes the removal of an implicit conversion to `const T&` if the
types mismatch. Making `less<T>` transparent in that case changes
overload resolution and makes it fail potentially.

Fixes #179319

GitOrigin-RevId: 9d2303103288f6110622644f78dbd26c8bcf28d5