libstdc++: Use allocator to construct std::stacktrace_entry objects

Because std::basic_stacktrace<A> is an allocator-aware container its
elements should be initialized using allocator_traits<A>::construct and
destroyed using allocator_traits<A>::destroy.

This adds new _M_clone and _M_assign helper functions to construct
elements correctly and uses those functions instead of calling
std::uninitialized_copy_n.

The _Impl::_M_destroy function needs to be passed an allocator to
destroy the elements correctly, so is replaced by _M_resize which can
also be used to trim the container to a smaller size.

Because destroying and creating std::stacktrace_entry objects is cheap,
the copy/move assignment operators can just destroy all existing
elements and use _Impl._M_clone or _Impl._M_assign to create new ones.

libstdc++-v3/ChangeLog:

	* include/std/stacktrace (basic_stacktrace): Use _Impl::_M_clone
	or _Impl::_M_assign to initialize elements in allocated storage.
	(basic_stacktrace::_M_clear()): Use _Impl::_M_resize instead of
	_Impl::_M_destroy.
	(basic_stacktrace::_Impl::_M_destroy()): Replace with ...
	(basic_stacktrace::_Impl::_M_resize(size_type, allocator&)): New
	function.
	(basic_stacktrace::_Impl::_M_push_back): Use _M_xclone. Construct
	new element using allocator.
	(basic_stacktrace::_Impl::_M_clone): New function.
	(basic_stacktrace::_Impl::_M_xclone): New function.
	(basic_stacktrace::_Impl::_M_assign): New function.

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