libstdc++: Begin lifetime of chars in constexpr std::string [PR103295]

libstdc++: Begin lifetime of chars in constexpr std::string [PR103295]

Clang gives errors for constexpr std::string because the memory returned
by std::allocator<T>::allocate does not contain any objects yet, and
attempting to set them using char_traits::assign or char_traits::copy
fails with:

assignment to object outside its lifetime is not allowed in a constant expression
              *__result = *__first;
                        ^
This adds code to std::char_traits to use std::construct_at to begin
lifetimes when called during constant evaluation. To support
specializations of std::basic_string that don't use std::char_traits
there is now another layer of wrapper around the allocator_traits, so
that the lifetime of characters is begun as soon as the memory is
allocated. By doing it in the char traits and allocator traits, the rest
of basic_string can ignore the problem.

While modifying char_traits::copy and char_traits::assign to begin
lifetimes for the constexpr cases, I also replaced their uses of
std::copy and std::fill_n respectively. That means we don't need
<bits/stl_algobase.h> for char_traits.

libstdc++-v3/ChangeLog:

	PR libstdc++/103295
	* include/bits/basic_string.h (_Alloc_traits): Replace typedef
	with struct for C++20 mode.
	* include/bits/basic_string.tcc (_M_replace): Use _Alloc_traits
	for allocation.
	* include/bits/char_traits.h (__gnu_cxx::char_traits::assign):
	Use std::_Construct during constant evaluation.
	(__gnu_cxx::char_traits::assign(CharT*, const CharT*, size_t)):
	Likewise. Replace std::fill_n with memset or manual loop.
	(__gnu_cxx::char_traits::copy): Likewise, replacing std::copy
	with memcpy.
	* include/ext/vstring.h: Include <bits/stl_algobase.h> for
	std::min.
	* include/std/string_view: Likewise.
	* testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc:
	Add constexpr test.