[Serialization] Distinguish between protocol/extension for types too. (#7794)

Replace an existing flag for cross-references to member types (that
wasn't getting much use) with one consistent with how we lookup
values. This fixes the case where someone actually has a useful type
as a member of a protocol extension, and that type gets referenced in
another module; Dispatch does exactly this.

Because you can currently only define typealiases in protocol
extensions, not new types, there's always a workaround for someone
hitting this issue: just use the underlying type.

https://bugs.swift.org/browse/SR-4076

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 = 320; // Last change: inherited protocols
+const uint16_t VERSION_MINOR = 321; // Last change: restrict to extension
 
 using DeclID = PointerEmbeddedInt<unsigned, 31>;
 using DeclIDField = BCFixed<31>;
@@ -1181,7 +1181,7 @@ namespace decls_block {
   using XRefTypePathPieceLayout = BCRecordLayout<
     XREF_TYPE_PATH_PIECE,
     IdentifierIDField, // name
-    BCFixed<1>         // only in nominal
+    BCFixed<1>         // restrict to protocol extension
   >;
 
   using XRefValuePathPieceLayout = BCRecordLayout<