To satisfy the CopyConstructible requirement a callable object stored in
a std::function must behave the same when copied from a const or
non-const source. If copying a non-const object doesn't produce an
equivalent copy then the behaviour is undefined. But we can make our
std::function more tolerant of such objects by ensuring we always copy
from a const lvalue.

Additionally use an if constexpr statement in the _M_get_pointer
function to avoid unnecessary instantiations in the discarded branch.

	* include/bits/std_function.h (_Base_manager::_M_get_pointer):
	Use constexpr if in C++17 mode.
	(_Base_manager::_M_clone(_Any_data&, const _Any_data&, true_type)):
	Copy from const object.
	* testsuite/20_util/function/cons/non_copyconstructible.cc: New.

Tested powerpc64le-linux, committed to trunk.