ObjCExport accessed `__NSCFBoolean` using
`objc_getClass("__NSCFBoolean")`. This class is a private API.
Remove this private API usage by getting the same class from
`[[NSNumber numberWithBool:YES] class]` instead.
Also remove `__NSCFBoolean` from the related error messages.
`__NSCFBoolean` didn't cause Kotlin-compiled apps to be rejected by
Apple, but this commit reduces the risk that this might happen in the
future, and addresses related user concerns.
^KT-62091