[Serialization] Preparation for giving swiftdoc its own version

The functionality change in this commit is that the control block in a
swiftdoc file is validated rather than just being ignored. Tests in
following commit.

include/swift/Serialization/ModuleFile.h

@@ -548,6 +548,11 @@ private:
   /// Returns false if there was an error.
   bool readCommentBlock(llvm::BitstreamCursor &cursor);
 
+  /// Loads data from #ModuleDocInputBuffer.
+  ///
+  /// Returns false if there was an error.
+  bool readModuleDocIfPresent();
+
   /// Recursively reads a pattern from \c DeclTypeCursor.
   llvm::Expected<Pattern *> readPattern(DeclContext *owningDC);