[serialization] Add support for classes.

Classes are exactly like structs except that they may have a base class.
However, this type will show up in the inheritance list. That means we
don't actually need to serialize it twice; we can just grab the base class
from the inheritance list.

Swift SVN r6133

lib/Serialization/ModuleFile.h

@@ -165,7 +165,13 @@ private:
   /// Creates an array of types from the given IDs.
   ///
   /// The returned array is owned by the ASTContext.
-  MutableArrayRef<TypeLoc> getTypes(ArrayRef<uint64_t> rawTypeIDs);
+  ///
+  /// \param rawTypeIDs An array of TypeID values, but using \c uint64_t for
+  ///                   compatibility with BitstreamReader.
+  /// \param[out] classType If non-null, and one of the type IDs resolves to a
+  ///                       class type, this will be set to that type.
+  MutableArrayRef<TypeLoc> getTypes(ArrayRef<uint64_t> rawTypeIDs,
+                                    Type *classType = nullptr);
 
   /// Reads members of a DeclContext from \c DeclTypeCursor.
   ///