Private suspend functions need to be generated as package-local, since
they are called from their continuations.
However, this means that they can be called from Java, which breaks
their private visibility.
Adding synthetic to them fixes the issue.
#KT-17584: Fixed
Sometimes instead of {POP, GETSTATIC Unit.INSTANCE, ARETURN} sequence
the codegen emits {CHECKCAST Unit, ARETURN} sequence, which breaks tail
call optimization. By replacing CHECKCAST with ARETURN we eliminate
this issue.
#KT-19790: Fixed
