[llvm] [MergeFunctions] Add support to run the pass over a set of function pointers (PR #111045)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

As-of latest changes this LGTM, I think this shouldn't impact existing code either since it's just adding a new way to invoke MergeFunctions - just pinging @nikic in case they either have opinions about this change or know someone who might (I don't see any maintainers that cover this file or recent contributors to this area).

https://github.com/llvm/llvm-project/pull/111045