support nested abstract clafers

[mantkiew]

abstract Component
    abstract Port

now, Port can be extended as:

Example 1:

WinController : Component
    cmd : Port
    abstract ExceptionPort : Port

NOTE: the compiler also allows declaring ExceptionPort at the top-level but only for abstract clafers like this

WinController : Component
    powerOut : ExceptionPort

abstract ExceptionPort : Port

however, in the IR, ExceptionPort is physically moved as a sibling of Port as in the Example 1.

Relevant test cases are:

• nestedInheritanceWithReference
• NestedInheritanceAndReference2
• gi67-extending-nested-abstract

[JLiangWaterloo]

Implemented abstract clafers nested under abstract clafers.

TODO: abstract clafers nested under concrete clafers.