* Fix Issue 13732 - non-member templates can use "template this"
* Ignore template this parameter inside a template as it could be a mixin
Note: Can't use goto to jump into scope(exit).
* Fix detecting aggregate template with TemplateThisParameter
* Remove parse changes
* Implement semantic checks for template this
* Remove error for explicit template this parameter not class or struct
* Merge error output, test mixin and remove static this for now
