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.