Refactor error logging for unknown function users

Author: wsmoses

src/llvm/transforms.jl

@@ -2529,9 +2529,9 @@ function removeDeadArgs!(mod::LLVM.Module, tm::LLVM.TargetMachine, post_gc_fixup
                     u = LLVM.user(u)
 		    if !isa(u, LLVM.CallInst)
                     	msg = sprint() do io
-			   println("Unknown user of fn: ", string(u))
-			   println("fn: ", string(fn))
-			   println("mod: ", string(parent(fn)))
+			   println(io, "Unknown user of fn: ", string(u))
+			   println(io, "fn: ", string(fn))
+			   println(io, "mod: ", string(parent(fn)))
 			end
 			throw(AssertionError(msg))
 		    end