libstdc++: Avoid using std::unique_ptr in <locale>

libstdc++: Avoid using std::unique_ptr in <locale>

std::wstring_convert and std::wbuffer_convert types are not copyable or
movable, and store a plain pointer without a deleter. That means a much
simpler type that just uses delete in its destructor can be used instead
of std::unique_ptr.

That avoids including and parsing all of <bits/unique_ptr.h> in every
header that includes <locale>. It also avoids instantiating
unique_ptr<C> and std::tuple<C*, default_delete<C>> when the conversion
utilities are used.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/bits/locale_conv.h (__detail::_Scoped_ptr): Define new
	RAII class template.
	(wstring_convert, wbuffer_convert): Use __detail::_Scoped_ptr
	instead of unique_ptr.