c++: Make __extension__ silence -Wlong-long pedwarns/warnings [PR121133]

c++: Make __extension__ silence -Wlong-long pedwarns/warnings [PR121133]

The PR13358 r0-92909 change changed the diagnostics on long long
in C++ (either with -std=c++98 or -Wlong-long), but unlike the
C FE we unfortunately warn even in the
__extension__ long long a;
etc. cases.  The C FE in that case in
disable_extension_diagnostics saves and clears not just
pedantic flag but also warn_long_long (and several others), while
C++ FE only temporarily disables pedantic.

The following patch makes it behave like the C FE in this regard,
though (__extension__ 1LL) still doesn't work because of the
separate lexing (and I must say I have no idea how to fix that).

Or do you prefer a solution closer to the C FE, cp_parser_extension_opt
saving the values into a bitfield and have another function to restore
the state (or use RAII)?

2025-07-30  Jakub Jelinek  <jakub@redhat.com>

	PR c++/121133
	* parser.cc (cp_parser_unary_expression): Adjust
	cp_parser_extension_opt caller and restore warn_long_long.
	(cp_parser_declaration): Likewise.
	(cp_parser_block_declaration): Likewise.
	(cp_parser_member_declaration): Likewise.
	(cp_parser_extension_opt): Add SAVED_LONG_LONG argument,
	save previous warn_long_long state into it and clear it
	for __extension__.

	* g++.dg/warn/pr121133-1.C: New test.
	* g++.dg/warn/pr121133-2.C: New test.
	* g++.dg/warn/pr121133-3.C: New test.
	* g++.dg/warn/pr121133-4.C: New test.

(cherry picked from commit fac66b476afccac607dbd9b4e971a42a04666387)