libstdc++: Do not use CTAD for _Utf32_view alias template

We were relying on P1814R0 (CTAD for alias templates) which isn't
supported by Clang. We can just not use CTAD and provide an explicit
template argument list for _Utf32_view.

Ideally we'd define a deduction guide for _Grapheme_cluster_view that
uses views::all_t to properly convert non-views to views, but all_t is
defined in <ranges> and we don't want to include all of that in
<bits/unicode.h>. So just make it require a view for now, which can be
cheaply copied.

Although it's not needed yet, it would also be more correct to
specialize enable_borrowed_range for the views in <bits/unicode.h>.

libstdc++-v3/ChangeLog:

	* include/bits/unicode.h (_Grapheme_cluster_view): Require view.
	Do not use CTAD for _Utf32_view.
	(__format_width, __truncate): Do not use CTAD.
	(enable_borrowed_range<_Utf_view<T, R>>): Define specialization.
	(enable_borrowed_range<_Grapheme_cluster_view<R>>): Likewise.