[Serialization] Handle XREFs to private types (#14352)

We can encounter these when the compiler modifies an inlinable
function to break apart a struct and the struct uses a private
type for one of its fields. It's questionable whether we /should/
handle this, but meanwhile this /is/ a non-intrusive fix that
preserves the performance of non-resilient libraries.

(That is, it appears this worked in Swift 4.0, though perhaps
not all of the same optimizations kicked in.)

https://bugs.swift.org/browse/SR-6874

include/swift/Serialization/ModuleFormat.h

@@ -54,7 +54,7 @@ const uint16_t VERSION_MAJOR = 0;
 /// in source control, you should also update the comment to briefly
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
-const uint16_t VERSION_MINOR = 397; // No resilience expansion in SILDeclRef
+const uint16_t VERSION_MINOR = 398; // Private discriminators for type xrefs
 
 using DeclIDField = BCFixed<31>;
 
@@ -1288,6 +1288,7 @@ namespace decls_block {
   using XRefTypePathPieceLayout = BCRecordLayout<
     XREF_TYPE_PATH_PIECE,
     IdentifierIDField, // name
+    IdentifierIDField, // private discriminator
     BCFixed<1>         // restrict to protocol extension
   >;