c++: remember pointer-to-member location

Jakub recently mentioned that a PTRMEM_CST has no location; let's give it a
location wrapper.

gcc/cp/ChangeLog:

	* typeck.c (build_x_unary_op): Set address location.
	(convert_member_func_to_ptr): Handle location wrapper.
	* pt.c (convert_nontype_argument): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/template/crash106.C: Adjust.
	* g++.dg/diagnostic/ptrtomem3.C: New test.