Nashorn code assumes NashornCallSiteDescriptor always

Nashorn code dynalink CallSiteDescriptor to be always NashornCallSiteDescriptor instance. This is true if Nashorn linkers are the only linkers used. With pluggable dynalink linkers, a linker may translate NashornCallSiteDescriptor created by Nashorn to either CallSiteDescriptor instance or instance of subclass of it. So, Nashorn code should avoid unconditionally casting CallSiteDescriptor to NashornCallSiteDescriptor.