c++: Add new warning options for C++ language mismatches

This adds new warning flags, enabled by default: -Wc++11-extensions,
-Wc++14-extensions, -Wc++17-extensions, -Wc++20-extensions, and
-Wc++23-extensions. The names of the flags are copied from Clang, which
already has similar options.

No new diagnostics are added, but the new OPT_Wxxx variables are used to
control existing pedwarns about occurences of new C++ constructs in code
using an old C++ standard dialect. This allows several existing warnings
that cannot currently be disabled to be controlled by the appropriate
-Wno-xxx flag. For example, it will now be possible to disable warnings
about using variadic templates in C++98 code, by using the new
-Wno-c++11-extensions option. This will allow libstdc++ headers to
disable those warnings unconditionally by using diagnostic pragmas, so
that they are not emitted even if -Wsystem-headers is used.

Some of the affected diagnostics are currently only given when
-Wpedantic is used. Now that we have a more specific warning flag, we
could consider making them not depend on -Wpedantic, and only on the new
flag. This patch does not do that, as it intends to make no changes to
what is accepted/rejected by default. The only effect should be that
the new option is shown when -fdiagnostics-show-option is active, and
that some warnings can be disabled by using the new flags (and for the
warnings that previously only dependend on -Wpedantic, it will now be
possible to disable just those warnings while still using -Wpedantic for
its other benefits).

gcc/c-family/ChangeLog:

	* c.opt (Wc++11-extensions, Wc++14-extensions)
	(Wc++17-extensions, Wc++20-extensions, Wc++23-extensions): New
	options.

gcc/cp/ChangeLog:

	* call.c (maybe_warn_array_conv): Use new warning option.
	* decl.c (mark_inline_variable, grokdeclarator): Likewise.
	* error.c (maybe_warn_cpp0x): Likewise.
	* parser.c (cp_parser_primary_expression)
	(cp_parser_unqualified_id)
	(cp_parser_pseudo_destructor_name)
	(cp_parser_lambda_introducer)
	(cp_parser_lambda_declarator_opt)
	(cp_parser_selection_statement)
	(cp_parser_init_statement)
	(cp_parser_decomposition_declaration)
	(cp_parser_function_specifier_opt)
	(cp_parser_static_assert)
	(cp_parser_namespace_definition)
	(cp_parser_using_declaration)
	(cp_parser_asm_definition)
	(cp_parser_ctor_initializer_opt_and_function_body)
	(cp_parser_initializer_list)
	(cp_parser_type_parameter_key)
	(cp_parser_member_declaration)
	(cp_parser_try_block)
	(cp_parser_std_attribute_spec): Likewise.
	* pt.c (check_template_variable): Likewise.

gcc/ChangeLog:

	* doc/invoke.texi (-Wno-c++11-extensions)
	(-Wno-c++14-extensions, -Wno-c++17-extensions)
	(-Wno-c++20-extensions, -Wno-c++23-extensions): Document
	new options.

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