Bug 289360 parse_type_DIE confused by DW_TAG_enumeration_type.

GCC allows incomplete enums as GNU extension.
http://gcc.gnu.org/onlinedocs/gcc/Incomplete-Enums.html
These are marked as DW_AT_declaration and won't have a size.
They can only be used in declaration or as pointer types.
You can't allocate variables or storage using such an enum type.
So don't require a size for such enum types.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13433 a5019735-40e9-0310-863c-91ae7b9d1cf9