libstdc++: Use RAII in <bits/stl_uninitialized.h>

This adds an _UninitDestroyGuard class template, similar to
ranges::_DestroyGuard used in <bits/ranges_uninitialized.h>. This allows
us to remove all the try-catch blocks and rethrows, because any required
cleanup gets done in the guard destructor.

libstdc++-v3/ChangeLog:

	* include/bits/stl_uninitialized.h (_UninitDestroyGuard): New
	class template and partial specialization.
	(__do_uninit_copy, __do_uninit_fill, __do_uninit_fill_n)
	(__uninitialized_copy_a, __uninitialized_fill_a)
	(__uninitialized_fill_n_a, __uninitialized_copy_move)
	(__uninitialized_move_copy, __uninitialized_fill_move)
	(__uninitialized_move_fill, __uninitialized_default_1)
	(__uninitialized_default_n_a, __uninitialized_default_novalue_1)
	(__uninitialized_default_novalue_n_1, __uninitialized_copy_n)
	(__uninitialized_copy_n_pair): Use it.