Bugfix: Do not mangle overridden non-overloaded virtual function names

[xermicus]

Closes #1623

The basic idea of the algorithm checking whether a function name is eligible for mangling or not is to mangle the name if that function is publicly callable but the functions name appears multiple time in the contract. But this doesn't account for virtual functions also appearing more than one time in the same contract if they are overridden. With this PR, we bail early if the function we are checking overrides, marking only the single one non-overriding implementation as eligible for mangling. Consequently, functions which override but do not overload are no longer unnecessarily mangled.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.57%. Comparing base (5cab937) to head (a9b7e02).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1624      +/-   ##
==========================================
- Coverage   88.58%   88.57%   -0.01%     
==========================================
  Files         150      150              
  Lines       69330    69330              
==========================================
- Hits        61415    61412       -3     
- Misses       7915     7918       +3     

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[xermicus]

Codecov says -0.01% coverage on the project however the diff hit 100%. I'm gonna merge