libstdc++: add #pragma diagnostic

The use of #pragma GCC system_header in libstdc++ has led to bugs going
undetected for a while due to the silencing of compiler warnings that would
have revealed them promptly, and also interferes with warnings about
problematic template instantiations induced by user code.

But removing it, or even compiling with -Wsystem-header, is also problematic
due to warnings about deliberate uses of extensions.

So this patch adds #pragma GCC diagnostic as needed to suppress these
warnings.

The change to acinclude.m4 changes -Wabi to warn only in comparison to ABI
19, to avoid lots of warnings that we now mangle concept requirements, which
are in any case still experimental.  I checked for any other changes against
ABI v15, and found only the <format> lambda mangling, which we can ignore.

This also enables -Wsystem-headers while building the library, so we see any
warnings not silenced by these #pragmas.

libstdc++-v3/ChangeLog:

	* include/bits/algorithmfwd.h:
	* include/bits/allocator.h:
	* include/bits/codecvt.h:
	* include/bits/concept_check.h:
	* include/bits/cpp_type_traits.h:
	* include/bits/hashtable.h:
	* include/bits/iterator_concepts.h:
	* include/bits/ostream_insert.h:
	* include/bits/ranges_base.h:
	* include/bits/regex_automaton.h:
	* include/bits/std_abs.h:
	* include/bits/stl_algo.h:
	* include/c_compatibility/fenv.h:
	* include/c_compatibility/inttypes.h:
	* include/c_compatibility/stdint.h:
	* include/ext/concurrence.h:
	* include/ext/type_traits.h:
	* testsuite/ext/type_traits/add_unsigned_floating_neg.cc:
	* testsuite/ext/type_traits/add_unsigned_integer_neg.cc:
	* testsuite/ext/type_traits/remove_unsigned_floating_neg.cc:
	* testsuite/ext/type_traits/remove_unsigned_integer_neg.cc:
	* include/bits/basic_ios.tcc:
	* include/bits/basic_string.tcc:
	* include/bits/fstream.tcc:
	* include/bits/istream.tcc:
	* include/bits/locale_classes.tcc:
	* include/bits/locale_facets.tcc:
	* include/bits/ostream.tcc:
	* include/bits/regex_compiler.tcc:
	* include/bits/sstream.tcc:
	* include/bits/streambuf.tcc:
	* configure: Regenerate.
	* include/bits/c++config:
	* include/c/cassert:
	* include/c/cctype:
	* include/c/cerrno:
	* include/c/cfloat:
	* include/c/climits:
	* include/c/clocale:
	* include/c/cmath:
	* include/c/csetjmp:
	* include/c/csignal:
	* include/c/cstdarg:
	* include/c/cstddef:
	* include/c/cstdio:
	* include/c/cstdlib:
	* include/c/cstring:
	* include/c/ctime:
	* include/c/cwchar:
	* include/c/cwctype:
	* include/c_global/climits:
	* include/c_global/cmath:
	* include/c_global/cstddef:
	* include/c_global/cstdlib:
	* include/decimal/decimal:
	* include/ext/rope:
	* include/std/any:
	* include/std/charconv:
	* include/std/complex:
	* include/std/coroutine:
	* include/std/format:
	* include/std/iomanip:
	* include/std/limits:
	* include/std/numbers:
	* include/tr1/functional:
	* include/tr1/tuple:
	* include/tr1/type_traits:
	* libsupc++/compare:
	* libsupc++/new: Add #pragma GCC diagnostic to suppress
	undesired warnings.
	* acinclude.m4: Change -Wabi version from 2 to 19.

gcc/ChangeLog:

	* ginclude/stdint-wrap.h: Add #pragma GCC diagnostic to suppress
	undesired warnings.
	* gsyslimits.h: Likewise.