Implement LWG 2769, Redundant const in the return type of any_cast(const any&).

Implement LWG 2769, Redundant const in the return type of any_cast(const any&).

Implement LWG 2769, Redundant const in the return type of
any_cast(const any&).
* include/std/any (_AnyCast): New.
(any_cast(const any&)): Use it and add an explicit cast for return.
(any_cast(any&)): Likewise.
(any_cast(any&&)): Likewise.
* testsuite/20_util/any/misc/any_cast.cc: Add a test for a type
that has an explicit copy constructor.
*testsuite/20_util/any/misc/any_cast_neg.cc: Adjust.

From-SVN: r243739