Set varobj->path_expr in varobj_get_path_expr

Set varobj->path_expr in varobj_get_path_expr

It seems like different languages are doing this differently (e.g.
C and Ada). For C, var->path_expr is set inside c_path_expr_of_child.
The next time the value is requested, is it therefore not recomputed.
Ada does not set this field, but just returns the value. Since the field
is never set, the value is recomputed every time it is requested.

This patch makes it so that path_expr_of_child's only job is to compute
the path expression, not save/cache the value. The field is set by the
varobj common code.

gdb/ChangeLog:

	* varobj.c (varobj_get_path_expr): Set var->path_expr.
	* c-varobj.c (c_path_expr_of_child): Set local var instead of
	child->path_expr.
	(cplus_path_expr_of_child): Same.