[serialization] Write substitution conformances as references when possible.

More importantly, when writing substitution conformances /within the same
module/, use an "incomplete" form of the NormalProtocolConformance layout
that doesn't include any of the substitutions or defaulted definitions.
This avoids a serialization cycle when the witness for a protocol itself
ends up conforming to the protocol.

I couldn't come up with a reduced test cases, but both cases filed by Dave
now work.

<rdar://problem/16468715>

Swift SVN r15912

include/swift/Serialization/ModuleFile.h

@@ -307,12 +307,12 @@ private:
   /// If the record at the cursor is not a pattern, returns null.
   Pattern *maybeReadPattern();
 
-  /// Read the underlying conformance for a specialized or inherited
-  /// protocol conformance.
+  /// Read a referenced conformance, such as the underlying conformance for a
+  /// specialized or inherited protocol conformance.
   ProtocolConformance *
-  readUnderlyingConformance(ProtocolDecl *proto,
+  readReferencedConformance(ProtocolDecl *proto,
                             serialization::DeclID typeID,
-                            serialization::IdentifierID moduleID,
+                            serialization::ModuleID moduleID,
                             llvm::BitstreamCursor &Cursor);
 
   GenericParamList *maybeGetOrReadGenericParams(serialization::DeclID contextID,