[RelLookupTableConverter] Avoid querying TTI for declarations

[RelLookupTableConverter] Avoid querying TTI for declarations

This code queries TTI on a single function, which is considered to
be representative. This is a bit odd, but probably fine in practice.

However, I think we should at least avoid querying declarations,
which e.g. will generally lack target attributes, and for which
we don't seem to ever query TTI in other places.