[Serialization] Only serialize SIL function generic env. with the body

When serializing a SIL function declaration only (no body), suppress
the generic environment as well. This was the case previously, but I
regressed it when moving the serialization of the generic environment
to ID-based serialization in 69cc9f4.

Fixes rdar://problem/29905180.

Author: DougGregor

lib/Serialization/SerializeSIL.cpp

@@ -350,7 +350,7 @@ void SILSerializer::writeSILFunction(const SILFunction &F, bool DeclOnly) {
 
   // If we have a body, we might have a generic environment.
   GenericEnvironmentID genericEnvID = 0;
-  if (!F.isExternalDeclaration())
+  if (!NoBody)
     genericEnvID = S.addGenericEnvironmentRef(F.getGenericEnvironment());
 
   DeclID clangNodeOwnerID;