Reapply "Revert "[func-sig-opts] If we know all callsites of a function, delete the old unoptimized function instead of creating a thunk.""

This reverts commit r23725. This time with fixes so we don't hit that
assertion.

Swift SVN r23762

include/swift/SIL/SILFunction.h

@@ -154,6 +154,10 @@ public:
     return LoweredType;
   }
 
+  bool canBeDeleted() const {
+    return !getRefCount() && !isZombie();
+  }
+
   /// Return the number of entities referring to this function (other
   /// than the SILModule).
   unsigned getRefCount() const { return RefCount; }