re PR c++/16853 (pointer-to-member initialization from incompatible one accepted)

re PR c++/16853 (pointer-to-member initialization from incompatible one accepted)

	PR c++/16853
	* call.c (standard_conversion): Do not accept conversions between
	pointers to members if the class types are unrelated.

	PR c++/16618
	* parser.c (cp_parser_builtin_offsetof): Cast to "const volatile
	char &" instead of just "char &".

	PR c++/16870
	* pt.c (tsubst): Just return the unknown_type_node.

	PR c++/16853
	* g++.dg/init/ptrmem1.C: New test.

	PR c++/16618
	* g++.dg/parse/offsetof5.C: New test.

	PR c++/16870
	* g++.dg/template/overload3.C: New test.

From-SVN: r85840