c++: can't eval PTRMEM_CST in incomplete class [PR107574]

c++: can't eval PTRMEM_CST in incomplete class [PR107574]

Here we're attempting to evaluate a PTRMEM_CST in a class that hasn't
been completed yet, but that doesn't work:

        /* We can't lower this until the class is complete.  */
        if (!COMPLETE_TYPE_P (DECL_CONTEXT (member)))
          return cst;

and then this unlowered PTRMEM_CST is used as EXPR in

    tree op1 = build_nop (ptrdiff_type_node, expr);

and we crash in a subsequent cp_fold_convert which gets type=ptrdiff_type_node,
expr=PTRMEM_CST and does

  else if (TREE_CODE (expr) == PTRMEM_CST
           && same_type_p (TYPE_PTRMEM_CLASS_TYPE (type),
                           PTRMEM_CST_CLASS (expr)))

where TYPE_PTRMEM_CLASS_TYPE (type) is going to crash since the type
is ptrdiff_type_node.  We could just add a TYPE_PTRMEM_P check before
accessing TYPE_PTRMEM_CLASS_TYPE but I think it's nicer to explain why
we couldn't evaluate the expression.

	PR c++/107574

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_constant_expression): Emit an error when
	a PTRMEM_CST cannot be evaluated.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/ptrmem-cst1.C: New test.

(cherry picked from commit de81e06273c613d7e06cbe2c8d9e72826c638056)