Revert "Performance: Don't look for implicit interface implementations if the language doesn't support it."

This reverts commit b90f222e.

Use GetMemberGroup() instead of GetSymbolInfo().CandidateSymbols to get all method overloads and not just the best fitting ones.

Added code comments for bug report #25135 and added BC36625 as supported diagnostic in anticipation of the bug fix.

Refactored GetAllMethodSymbolsOfPartialParts and switch the ordering, so the first element in the array is always the implementation.