[IR] Track users of comdats

Track all GlobalObjects that reference a given comdat, which allows
determining whether a function in a comdat is dead without scanning
the whole module.

In particular, this makes filterDeadComdatFunctions() have complexity
O(#DeadFunctions) rather than O(#SymbolsInModule), which addresses
half of the compile-time issue exposed by D115545.

Differential Revision: https://reviews.llvm.org/D115864