[libc++] Allow building the dylib with the observe assertion semantic

[ldionne]

In #149459, we added assertion semantics (experimentally). One of these semantics is the observe semantic, which requires linking against libc++experimental.a. That means that we currently can't build the dylib itself with the observe semantic, since that would create a cyclic dependency (libc++experimental.a requires the dylib for various things, and libc++.dylib requires libc++experimental.a for the definition of the logging function).

This issue would go away on its own if we made the logging function (and the assertion semantics) non-experimental, because then we'd move the logging function to the dylib.