Currently FirThisReceiverExpression of instance methods are translated
to references of the class' thisReceiver,
not the method's dispatch receiver,
which causes problems with IrFrameMap::typeOf,
as the class' thisReceiver is not in the typeMap.
This commit translates non-qualified "this" references of
instance methods to references of the methods' dispatch receiver.
This is done primarily for JVM interoperability, otherwise it's impossible to
inherit from CharSequence there. On JS subSequence at the moment just invokes
substring.
#KT-5956 Fixed
Juan Chen