Supplemental change for Issue 1983 - Delegates violate const

http://d.puremagic.com/issues/show_bug.cgi?id=1983

After fixing bug 1983, `ReturnType`, `functionAttributes`, and other function type trait templates will not work for overloaded function symbol. For `@property` functions, using `typeof(propfunc)` idiom would be better.

And, recently merged `wrap` and `unwrap` have wrong implementation against fixing bug 1983. I need fixing the incorrect code at the same time.

https://github.com/D-Programming-Language/dmd/pull/2130 requires this change.