swift234

[playgrounds] Implement _swift_isClass(x: Any) -> Bool.

This is acting as a temporary work around until the more general
getTypeKind() is finalized from an API perspective.

The reason that removing this now instead of waiting makes sense is:

1. The previous way playgrounds was performing this check was using the runtime
c++ implementation of swift_isClassOrObjCExistential via @asmname. We
want to move away from the @asmname approach in general since it is
brittle since it leads to loose cupling of dependencies.

2. The @asmname version in playgrounds caused the deserializer to assert since
the c++ version of swift_isClassOrObjCExistential() does not match the signature
of the swift declaration of swift_isClassOrObjCExistential().

This allows us to avoid the aforementioned issues and make forward
progress on re-enabling the playground logger with optimization.

rdar://18841292

Swift SVN r23307