r1671: make [relative] pointers in idl much more generic, treating them just

r1671: make [relative] pointers in idl much more generic, treating them just
like normal pointers in most cases. This means we can now support
relative pointers to unions, builtin types etc, whereas we could only
previously support relative pointers to structures.

metze needs this for the PAC decoding.
(This used to be commit 0d063725e12f51375b7d0be55a19072a9a54e7e6)