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.

gcc/cp/ChangeLog

+2020-02-02  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93530 - ICE on invalid alignas in a template.
+	* decl.c (grokdeclarator): Call cplus_decl_attributes instead of
+	decl_attributes.
+
 2020-01-31  Jason Merrill  <jason@redhat.com>
 
 	PR c++/86216

gcc/cp/decl.c

@@ -12318,7 +12318,7 @@ grokdeclarator (const cp_declarator *declarator,
 
 		 The optional attribute-specifier-seq appertains to
 		 the function type.  */
-	      decl_attributes (&type, attrs, 0);
+	      cplus_decl_attributes (&type, attrs, 0);
 
 	    if (raises)
 	      type = build_exception_variant (type, raises);

gcc/testsuite/ChangeLog

+2020-02-02  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93530 - ICE on invalid alignas in a template.
+	* g++.dg/cpp0x/alignas18.C: New test.
+
 2020-02-02  Iain Sandoe  <iain@sandoe.co.uk>
 
 	* gcc.target/powerpc/darwin-abi-12.c: Add '-fcommon' to the

gcc/testsuite/g++.dg/cpp0x/alignas18.C

+// PR c++/93530 - ICE on invalid alignas in a template.
+// { dg-do compile { target c++11 } }
+
+template <typename T> struct S {
+  using U = S;
+  // FIXME: This is ill-formed; see PR90847.
+  void fn() alignas(U::X);
+};