libstdc++: Refactor bound arguments storage for bind_front/back

libstdc++: Refactor bound arguments storage for bind_front/back

This patch refactors the implementation of bind_front and bind_back to avoid
using std::tuple for argument storage. Instead, bound arguments are now:
* stored directly if there is only one,
* within a dedicated _Bound_arg_storage otherwise.

_Bound_arg_storage is less expensive to instantiate and access than std::tuple.
It can also be trivially copyable, as it doesn't require a non-trivial assignment
operator for reference types. Storing a single argument directly provides similar
benefits compared to both one element tuple or _Bound_arg_storage.

_Bound_arg_storage holds each argument in an _Indexed_bound_arg base object.
The base class is parameterized by both type and index to allow storing
multiple arguments of the same type. Invocations are handled by _S_apply_front
amd _S_apply_back static functions, which simulate explicit object parameters.
To facilitate this, the __like_t alias template is now unconditionally available
since C++11 in bits/move.h.

libstdc++-v3/ChangeLog:

	* include/bits/move.h (std::__like_impl, std::__like_t): Make
	available in c++11.
	* include/std/functional (std::_Indexed_bound_arg)
	(std::_Bound_arg_storage, std::__make_bound_args): Define.
	(std::_Bind_front, std::_Bind_back): Use _Bound_arg_storage.
	* testsuite/20_util/function_objects/bind_back/1.cc: Expand
	test to cover cases of 0, 1, many bound args.
	* testsuite/20_util/function_objects/bind_back/111327.cc: Likewise.
	* testsuite/20_util/function_objects/bind_front/1.cc: Likewise.
	* testsuite/20_util/function_objects/bind_front/111327.cc: Likewise.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>