c++: fix ICE in convert_nontype_argument [PR116384]

c++: fix ICE in convert_nontype_argument [PR116384]

Here we ICE since r14-8291 in C++11/C++14 modes.  Fortunately
this is an easy one.

The important bit of r14-8291 is this:

@@ -20056,9 +20071,12 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
        RETURN (retval);
      }
    if (IMPLICIT_CONV_EXPR_NONTYPE_ARG (t))
-     /* We'll pass this to convert_nontype_argument again, we don't need
-        to actually perform any conversion here.  */
-     RETURN (expr);
+     {
+       tree r = convert_nontype_argument (type, expr, complain);
+       if (r == NULL_TREE)
+         r = error_mark_node;
+       RETURN (r);
+     }

which obviously means that instead of returning right away we go
to convert_nontype_argument.  When type is error_mark_node and we're
in C++17, in convert_nontype_argument we go down this path:

      else if (INTEGRAL_OR_ENUMERATION_TYPE_P (type)
               || cxx_dialect >= cxx17)
        {
          expr = build_converted_constant_expr (type, expr, complain);
          if (expr == error_mark_node)
            return (complain & tf_error) ? NULL_TREE : error_mark_node;
	  // ...
	}

but pre-C++17, we take a different route and end up crashing on
gcc_unreachable.

It would of course also work to check for error_mark_node early in
build_converted_constant_expr.

	PR c++/116384

gcc/cp/ChangeLog:

	* pt.cc (tsubst_expr) <case IMPLICIT_CONV_EXPR>: Bail if tsubst
	returns error_mark_node.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/vt-116384.C: New test.