add completion for members of anonymous unions and structs

This adds completion for members of anonymous unions and structs. The corresponding bug report for Visual D is here: https://d.puremagic.com/issues/show_bug.cgi?id=11894

Unfortunately, there are still problems with resolving an expression using a member of the anonymous union, very much like for anonymous enums (e.g. no tooltip). The tooltip on the member itself shows two consecutive dots, e.g. struct_name..enumerator_value.