Implement the missing bits of LWG 2769

Implement the missing bits of LWG 2769

* include/std/any (any_cast(const any&)): Add static_assert.
(any_cast(any&)): Likewise.
(any_cast(any&&)): Likewise, and remove the handling
for copyable-but-not-movable type.
* testsuite/20_util/any/misc/any_cast.cc: Adjust.
* testsuite/20_util/any/misc/any_cast_neg.cc: Likewise, and
add new tests.

From-SVN: r258031