Fix issues with properties from interfaces

[juliusikkala]

Fixes #7232.

1. A crash was caused by the parser, as it tried to use declRefExpr, which is always nullptr in the else branch. It seems that candidate was meant instead, and declRefExpr is copy-paste residue there. (Also TIL how the scopes of variables introduced in an if-condition work in C++, it's very weird that this ever even compiled :D)
2. The issue with the array index not being recognized as an integer was caused by the reference to the value property creating an OverloadExpr. CheckExpr is needed to resolve that overload before calling isScalarIntegerType.