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

We do not allow external declarations with shared visibility. This commit makes
the serializer translate shared_external linkage to public_external because the
serialized functions will be available at runtime.

rdar://21989088

Author: nadavrot

lib/Serialization/SerializeSIL.cpp

@@ -263,6 +263,10 @@ 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(