This commit includes the basic Java stubs generation infrastructure and
the corresponding tests. The main entry point is called
Kapt4StubGenerator. Calls to it from production code will be added in a
separate commit.
#KT-51982
Behavior differs between JVM and JVM_IR backends here because in JVM,
the class descriptor comes from the frontend, and its modality for enum
is never final. For JVM IR, the class descriptor is based on IrClass,
whose modality is sometimes final for enum, presumably because it's
easier for backends (see `ClassGenerator.getEffectiveModality`).
#KT-49682
Pavel Mikhailovskii