Revert "[Serialization] Fix a bug in the serialization of shared functions."

This reverts commit 422d466.

Jordan said that this change is incorrect. I am reverting the patch and plan to
investigate why we are deserializing shared_external functions with no body.

Author: nadavrot

lib/Serialization/SerializeSIL.cpp

@@ -263,10 +263,6 @@ void SILSerializer::writeSILFunction(const SILFunction &F, bool DeclOnly) {
   // as available externally.
   if (NoBody) {
     Linkage = addExternalToLinkage(Linkage);
-
-    // We do not allow external declarations with shared visibility.
-    if (hasSharedVisibility(Linkage))
-      Linkage = SILLinkage::PublicExternal;
   }
 
   SILFunctionLayout::emitRecord(