attribs: Fix wrong error with -Wno-attribute=A::b [PR103649]

My patch to implement -Wno-attribute=A::b caused a bogus error when
parsing

  [[foo::bar(1, 2)]];

when -Wno-attributes=foo::bar was specified on the command line, because
when we create a fake foo::bar attribute and insert it into our attribute
table, it is created with max_length == 0 which doesn't allow any args.
That is wrong -- we know nothing about the attribute, so we shouldn't
require any specific number of arguments.  And since unknown attributes
can be rather complex (see for example omp::{directive,sequence}), we
must skip parsing their arguments.  To that end, I'm using max_length
with value -2.

Also let's not warn about things like

  [[vendor::assume(true)]];

because they may have some meaning (this is reminiscent of C++ Portable
Assumptions).

	PR c/103649

gcc/ChangeLog:

	* attribs.c (handle_ignored_attributes_option): Create the fake
	attribute with max_length == -2.
	(attribute_ignored_p): New overloads.
	* attribs.h (attribute_ignored_p): Declare them.
	* tree-core.h (struct attribute_spec): Document that max_length
	can be -2.

gcc/c/ChangeLog:

	* c-decl.c (c_warn_unused_attributes): Don't warn for
	attribute_ignored_p.
	* c-parser.c (c_parser_std_attribute): Skip parsing of the attribute
	arguments when the attribute is ignored.

gcc/cp/ChangeLog:

	* parser.c (cp_parser_declaration): Don't warn for attribute_ignored_p.
	(cp_parser_std_attribute): Skip parsing of the attribute
	arguments when the attribute is ignored.

gcc/testsuite/ChangeLog:

	* c-c++-common/Wno-attributes-6.c: New test.