Get *=default*ed default constructors

Get *=default*ed default constructors

Summary: Defaulted (*=default*ed) default constructors are better
because they can be constexpr and/or noexcept when C++ Standard
thinks it is right to do so. And they remain user-declared rather
than user-provided. Regarding *=default*ed default constructor,
benifit is that the work might be done during compilation and we
might not need to worry about exception paths. And for destructors,
apart from that that =defaulted definition is in sync with defaulted
default constructor we might discover that in some cases "() {}" might
be ill-formed when "= default;" compiltion indicates so. If =defaulted
definition for destructor doesn't do any harm then why not go for it.

Closes #216

Reviewed By: @yfeldblum

Differential Revision: D2145322

Pulled By: @sgolemon