[backport #1727] SI-7359 cyclic nested java class

The original commit message (from 54a84a36d5):

SI-6548 reflection correctly enters jinners

When completing Java classes, runtime reflection enumerates their
fields, methods, constructors and inner classes, loads them and
enters them into either the instance part (ClassSymbol) or the
static part (ModuleSymbol).

However unlike fields, methods and constructors, inner classes don't
need to be entered explicitly - they are entered implicitly when
being loaded.

This patch fixes the double-enter problem, make sure that enter-on-load
uses the correct owner, and also hardens jclassAsScala against double
enters that can occur in a different scenario.