[trivial] fix style nitpicks
fix Issue 10658 - Cannot merge template overload set by using alias declaration

- Add OverDeclaration and see it in overloadApply.
- Use OverDeclaration for template overload set merging.
- Check OverDeclaration in DotVarExp and VarExp.
Merge pull request #2417 from 9rnsr/fix10658

Issue 10658 - Cannot merge template overload set by using alias declaration