Simplify getConvertibleFieldAtPosition

[ghost]

Had some code review on #22143 but didn't want to delay it any longer. @Kingwl

• PropertyDeclaration#parent is always a ClassLikeDeclaration. (In other contexts we use a PropertyAssignment or PropertySignature node instead. I know it's hard to keep track of all of these...)
• There were some unnecessary checks: members is not an optional property (and besides, it must contain at least propertyDeclaration).
• The get*Info functions share a lot in common, so it's simpler to check declaration.kind only when necessary.