Support @deprecated() on overloaded __get__ for generic descriptors

[Viicos]

Fixes #18323.
Follow up on #18090.

Notes:

• In analyze_descriptor_access, there is a distinction between typ and descriptor_type:
  

  mypy/mypy/checkmember.py

  Lines 687 to 688 in 9ff9946

  	typ = map_instance_to_supertype(descriptor_type, dunder_get.info)
  	dunder_get_type = expand_type_by_instance(bound_method, typ)

  it seems that in our case though, map_instance_to_supertype returns descriptor_type unchanged. The reason I'm mentioning this is that expand_type_by_instance uses typ and not descriptor_type. In warn_deprecated_overload_item, we have access to descriptor_type (passed as selftype):
  

  mypy/mypy/checkmember.py

  Lines 726 to 728 in 9ff9946

  	mx.chk.warn_deprecated_overload_item(
  	dunder_get, mx.context, target=inferred_dunder_get_type, selftype=descriptor_type
  	)

  and in this changeset I call expand_type_by_instance(candidate, selftype). Is this going to be an issue?
• To comply with the type checker, I had to change the type annotation of selftype to Instance. Is this going to be an issue?

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅