c++: Fix up pedantic handling of alignas [PR110345]

c++: Fix up pedantic handling of alignas [PR110345]

The following patch on top of the PR110345 P2552R3 series
emits pedantic pedwarns for alignas appertaining to incorrect entities.
As the middle-end and attribute exclusions look for "aligned" attribute,
the patch transforms alignas into "internal "::aligned attribute (didn't
use [[aligned (x)]] so that people can't type it that way).

2024-12-18  Jakub Jelinek  <jakub@redhat.com>

	PR c++/110345
gcc/c-family/
	* c-common.h (attr_aligned_exclusions): Declare.
	(handle_aligned_attribute): Likewise.
	* c-attribs.cc (handle_aligned_attribute): No longer
	static.
	(attr_aligned_exclusions): Use extern instead of static.
gcc/cp/
	* cp-tree.h (enum cp_tree_index): Add CPTI_INTERNAL_IDENTIFIER.
	(internal_identifier): Define.
	(internal_attribute_table): Declare.
	* parser.cc (cp_parser_exception_declaration): Error on alignas
	on exception declaration.
	(cp_parser_std_attribute_spec): Turn alignas into internal
	ns aligned attribute rather than gnu.
	* decl.cc (initialize_predefined_identifiers): Initialize
	internal_identifier.
	* tree.cc (handle_alignas_attribute): New function.
	(internal_attributes): New variable.
	(internal_attribute_table): Likewise.
	* cp-objcp-common.h (cp_objcp_attribute_table): Add
	internal_attribute_table entry.
gcc/testsuite/
	* g++.dg/cpp0x/alignas1.C: Add dg-options "".
	* g++.dg/cpp0x/alignas2.C: Likewise.
	* g++.dg/cpp0x/alignas7.C: Likewise.
	* g++.dg/cpp0x/alignas21.C: New test.
	* g++.dg/ext/bitfield9.C: Expect a warning.
	* g++.dg/cpp2a/is-layout-compatible3.C: Add dg-options -pedantic.
	Expect a warning.