c++: Apply/diagnose attributes when instatiating ARRAY/POINTER/REFERENCE_TYPE [PR118787]

c++: Apply/diagnose attributes when instatiating ARRAY/POINTER/REFERENCE_TYPE [PR118787]

The following testcase IMO in violation of the P2552R3 paper doesn't
pedwarn on alignas applying to dependent types or alignas with dependent
argument.

tsubst was just ignoring TYPE_ATTRIBUTES.

The following patch fixes it for the POINTER/REFERENCE_TYPE and
ARRAY_TYPE cases, but perhaps we need to do the same also for other
types (INTEGER_TYPE/REAL_TYPE and the like).  I guess I'll need to
construct more testcases.

2025-03-05  Jakub Jelinek  <jakub@redhat.com>

	PR c++/118787
	* pt.cc (tsubst) <case ARRAY_TYPE>: Use return t; only if it doesn't
	have any TYPE_ATTRIBUTES.  Call apply_late_template_attributes.
	<case POINTER_TYPE, case REFERENCE_TYPE>: Likewise.  Formatting fix.

	* g++.dg/cpp0x/alignas22.C: New test.