Magic variable "current_basic_block" should return an IL basic block if the currently selected basic block is an IL basic block

In light of PR https://github.com/Vector35/binaryninja-api/pull/4765, I notice that when we select an LLIL basic block, the repr of it shows a weird mixture:

>>> current_basic_block
<BasicBlock: x86_64@0xa-0x10>

This is because the current_basic_block object is always the BasicBlock base class. We should really return the LLIL/MLIL/HLIL basic block when appropriate, and maybe remove the current_il_basic_block, since it will always return the same thing.

One problem of not returning a subclass of the BasicBlock is some properties do not work:

>>> current_basic_block.instruction_count
0

In contrast, if we do have a proper LLIL basic block object, we get the correct instruction count value:

>>> current_il_function.basic_blocks[0].instruction_count
7

Start by reviewing PR 4765 and the API entry points for current_basic_block, current_il_basic_block, and current_il_function. Reproduce the shown LLIL selection and instruction_count behavior; done means current_basic_block returns the appropriate IL basic-block subclass for LLIL, MLIL, or HLIL selections, with current_il_basic_block's behavior considered accordingly.