Try resolving the LHS only when it looks (PSI-wise) like it could represent a
type element. This, for example, allows "illegal selector" error to be reported
on weird expressions like '""?.""::class'.
Also remove expression text from the "illegal selector" diagnostic, it's not
needed and can screw up the error message if the text is too big
Despite the fact that the LHS of a double colon expression is now KtExpression
instead of KtUserType, we must still consider it a type for callable references
to work. Extract and reuse relevant parts of QualifiedExpressionResolver and
TypeResolver to support this behavior