Update the zombie function list if we create new functions

Dead function elimination deletes functions making them zombies. A zombie is a
function whose name only exists because debug info might still refer to it.

However, later passes of specialization might create a new function by the same
name again. We now have a zombie function (which is just an alias to a deleted
method stub) and a function definition.

No test case since I could not reduce this to a small test case.

rdar://24659988

lib/SIL/SILFunction.cpp

@@ -95,6 +95,8 @@ SILFunction::SILFunction(SILModule &Module, SILLinkage Linkage,
   else
     Module.functions.push_back(this);
 
+  Module.removeFromZombieList(Name);
+
   // Set our BB list to have this function as its parent. This enables us to
   // splice efficiently basic blocks in between functions.
   BlockList.Parent = this;