More fix for Issue 9091 - Using __traits(getMember) on template argument fails inside member function

http://d.puremagic.com/issues/show_bug.cgi?id=9091

Use `ThisExp` instead of `VarExp` to avoid interpretation on template arguments.

By #1340, I had thought bug 9091 was fixed, but it was not enough. In there, my test case reduction was overdone. It was lacked about member function.