c-family: Fix ICE on attribute with -fgnu-tm [PR94733]

find_tm_attribute was using TREE_PURPOSE to get the attribute name,
which is breaking now that we preserve the C++11-style attribute
format past decl_attributes.  So use get_attribute_name which can
handle both formats of attributes.

	PR c++/94733
	* c-attribs.c (find_tm_attribute): Use get_attribute_name instead of
	TREE_PURPOSE.

	* g++.dg/tm/attrib-5.C: New test.

gcc/c-family/ChangeLog

+2020-04-23  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94733
+	* c-attribs.c (find_tm_attribute): Use get_attribute_name instead of
+	TREE_PURPOSE.
+
 2020-04-14  Patrick Palka  <ppalka@redhat.com>
 
 	PR c++/85278

gcc/c-family/c-attribs.c

@@ -3314,7 +3314,7 @@ find_tm_attribute (tree list)
 {
   for (; list ; list = TREE_CHAIN (list))
     {
-      tree name = TREE_PURPOSE (list);
+      tree name = get_attribute_name (list);
       if (tm_attr_to_mask (name) != 0)
 	return name;
     }

gcc/testsuite/ChangeLog

+2020-04-23  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/94733
+	* g++.dg/tm/attrib-5.C: New test.
+
 2020-04-23  Jakub Jelinek  <jakub@redhat.com>
 
 	PR middle-end/94724

gcc/testsuite/g++.dg/tm/attrib-5.C

+// PR c++/94733
+// { dg-do compile { target c++11 } }
+// { dg-options "-fgnu-tm" }
+
+struct [[gnu::may_alias]] pe { };