[slang] Fix virtual interface comparison

[likeamahoney]

systemverilog LRM (for example IEEE 1800-2017 or IEEE 1800-2023) 25.9 clause says:

— Equality ( == ) and inequality ( != ) with the following:
• Another virtual interface of the same type
• An interface instance of the same type
• The special constant null

It follows that virtual interfaces can be compared with instances of the same type which is not supported by slang for now.

I fixed this at that PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.75%. Comparing base (cad562f) to head (4fc070b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #915   +/-   ##
=======================================
  Coverage   93.75%   93.75%           
=======================================
  Files         190      190           
  Lines       47784    47799   +15     
=======================================
+ Hits        44799    44814   +15     
  Misses       2985     2985           

Files	Coverage Δ
source/ast/expressions/OperatorExpressions.cpp	87.87% <100.00%> (+0.12%)	⬆️

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[MikePopoloski]

Thanks for the PR!