c++: Fix ICE on invalid alignas in a template [PR93530]

c++: Fix ICE on invalid alignas in a template [PR93530]

This fixes an ICE taking place in cp_default_conversion because we got
a SCOPE_REF that doesn't have a type and so checking
INTEGRAL_OR_UNSCOPED_ENUMERATION_TYPE_P (TREE_TYPE (exp)) will crash.
This happens since the recent Joseph's change in decl_attributes whereby
we don't skip C++11 attributes on types.

[dcl.align] is clear that alignas applied to a function is ill-formed.
That should be fixed, and we have PR90847 for that.  But I think a more
appropriate fix at this stage would be the following: in a template we
want to splice dependent attributes and save them for later, and by
doing so avoid this crash.

	PR c++/93530 - ICE on invalid alignas in a template.
	* decl.c (grokdeclarator): Call cplus_decl_attributes instead of
	decl_attributes.

	* g++.dg/cpp0x/alignas18.C: New test.