[Demangling] Avoid UB call to memcpy().

When appending to an empty `CharVector`, we were inadvertently relying
on `memcpy(NewObjects, NULL, 0)` being a no-op, whereas in practice
the C standard says it's UB because of the `NULL` pointer.

Fix by only calling `memcpy()` if we actually have bytes to copy.

rdar://114447171

Author: al45tair

include/swift/Demangling/Demangler.h

@@ -206,7 +206,8 @@ class NodeFactory {
     if (Growth < Capacity * 2)
       Growth = Capacity * 2;
     T *NewObjects = Allocate<T>(Capacity + Growth);
-    memcpy(NewObjects, Objects, OldAllocSize);
+    if (OldAllocSize)
+      memcpy(NewObjects, Objects, OldAllocSize);
     Objects = NewObjects;
     Capacity += Growth;
   }