c++: Pedwarn on invalid decl specifiers for for-range-declaration [PR84009]

c++: Pedwarn on invalid decl specifiers for for-range-declaration [PR84009]

https://eel.is/c++draft/stmt.ranged#2
says that in for-range-declaration only type-specifier or constexpr
can appear.  As the following testcases show, we've emitted some
diagnostics in most cases, but not for static/thread_local (the patch
handles __thread too) and register in the non-sb case.
For extern there was an error that it is both extern and has an
initializer (again, non-sb only, sb errors on extern).

The following patch diagnoses those cases with pedwarn.
I've used for-range-declaration in the diagnostics wording (there was
already a case of that for the typedef), so that in the future
we don't need to differentiate it between range for and expansion
statements.

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

	PR c++/84009
	* parser.cc (cp_parser_decomposition_declaration): Pedwarn
	on thread_local, __thread or static in decl_specifiers for
	for-range-declaration.
	(cp_parser_init_declarator): Likewise, and also for extern
	or register.

	* g++.dg/cpp0x/range-for40.C: New test.
	* g++.dg/cpp0x/range-for41.C: New test.
	* g++.dg/cpp0x/range-for42.C: New test.
	* g++.dg/cpp0x/range-for43.C: New test.