We need to prevent stub kinds used just to represent dead elements to be
displayed in the GUI and CL.
We do so by introducing a DeadKind which expands their targets to the
empty list.
This commit introduces a reusable function to compact
pipeline::TargetLists that represent functions.
Initially this reasoning was only needed by TaggedFunctionKind, but now
also FunctionStringMap needs it, and potentially others in the future,
so it makes sense to keep only a single implementation for it instead of
duplicating code (and likely bugs).
Massimo Fioravanti