libstdc++: Define std::string::resize_and_overwrite for C++11 and COW string

There are several places in the library where we can improve performance
using resize_and_overwrite so it's inconvenient only being able to use
it in C++23 mode, and only for cxx11 strings. This adds it for COW
strings, and also adds __resize_and_overwrite as an extension for C++11
mode.

The new __resize_and_overwrite is available for C++11 and later, so
within the library we can use that consistently even in C++23.  In order
to avoid making a copy (which might not be possible for non-copyable,
non-movable types) the callable is passed to resize_and_overwrite as an
lvalue reference.  Unlike wrapping it in std::ref(op) this ensures that
invoking it as std::move(op)(n, p) will use the correct value category.
It also avoids any overhead that would be added by wrapping it in a
lambda like [&op](auto p, auto n) { return std::move(op)(p, n); }.

Adjust std::format to use the new __resize_and_overwrite, which we can
assume exists because we only use std::basic_string<char> and
std::basic_string<wchar_t>, so no program-defined specializations.

The uses in <experimental/internet> cannot be replaced, because those
are type-dependent on an Allocator template parameter, which could mean
they use program-defined specializations of std::basic_string that don't
have the __resize_and_overwrite extension.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (__resize_and_overwrite): New
	function.
	* include/bits/basic_string.tcc (__resize_and_overwrite): New
	function.
	(resize_and_overwrite): Simplify by using reserve instead of
	growing the string manually. Adjust for C++11 compatibility.
	* include/bits/cow_string.h (resize_and_overwrite): New
	function.
	(__resize_and_overwrite): New function.
	* include/bits/version.def (__cpp_lib_string_resize_and_overwrite):
	Do not depend on cxx11abi.
	* include/bits/version.h: Regenerate.
	* include/std/format (__formatter_fp::_S_resize_and_overwrite):
	Remove.
	(__formatter_fp::format, __formatter_fp::_M_localize): Use
	__resize_and_overwrite instead of _S_resize_and_overwrite.
	* testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc:
	Adjust for C++11 compatibility when included by ...
	* testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite_ext.cc:
	New test.

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