This consolidates the three static archives for extensions into one, so
that -lstdc++exp can be used to provide the definitions of all unstable
library features.

The libstdc++_libbacktrace.a archive is now just a "noinst" convenience
library that is only used during the build, not installed. Its contents
are added to libstdc++exp.a, along with the new non-inline definitions
of std::stacktrace symbols.

The libstdc++fs.a archive is still installed, but its contents are
duplicated in libstdc++exp.a now. This means -lstdc++exp can be used
instead of -lstdc++fs. For targets using the GNU linker we should
consider replacing libstdc++fs.a with a linker script that does
INPUT(libstdc++exp.a).

The tests for <experimental/filesystem> could be changed to use
-lstdc++exp instead of -lstdc++fs, which would allow removing
src/filesystem/.libs from the LDFLAGS in scripts/testsuite_flags.in,
but that can be done at a later date.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_CONFIGURE): Add c++23 directory.
	* configure: Regenerate.
	* doc/html/manual/*: Regenerate.
	* doc/xml/manual/using.xml: Update documentation on linking.
	* include/std/stacktrace: Remove declarations of libbacktrace
	APIs.
	(stacktrace_entry::_S_err_handler, stacktrace_entry::_S_init):
	Remove.
	(stacktrace_entry::_Info): New class.
	(stacktrace_entry::_M_get_info): Use _Info.
	(__stacktrace_impl): New class.
	(basic_stacktrace): Derive from __stacktrace_impl.
	(basic_stacktrace::current): Use __stacktrace_impl::_S_current.
	* scripts/testsuite_flags.in: Adjust LDFLAGS to find
	libstdc++exp instead of libstdc++_libbacktrace.
	* src/Makefile.am (SUBDIRS): Add c++23 directory.
	* src/Makefile.in: Regenerate.
	* src/c++20/Makefile.am: Fix comment.
	* src/c++20/Makefile.in: Regenerate.
	* src/c++23/Makefile.am: New file.
	* src/c++23/Makefile.in: New file.
	* src/c++23/stacktrace.cc: New file with definitions of
	stacktrace_entry::_Info and __stacktrace_impl members.
	* src/experimental/Makefile.am: Use LIBADD to include other
	libraries.
	* src/experimental/Makefile.in: Regenerate.
	* src/libbacktrace/Makefile.am: Use noinst_LTLIBRARIES.
	* src/libbacktrace/Makefile.in: Regenerate.
	* testsuite/19_diagnostics/stacktrace/current.cc: Adjust
	dg-options to use -lstdc++exp.
	* testsuite/19_diagnostics/stacktrace/entry.cc: Likewise.
	* testsuite/19_diagnostics/stacktrace/stacktrace.cc: Likewise.
	* testsuite/23_containers/vector/debug/assign4_backtrace_neg.cc:
	Likewise.