[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.

Author: jrose-apple

Diff for: include/swift/Serialization/DeclTypeRecordNodes.def

@@ -77,7 +77,7 @@
 #endif
 
 // These IDs must \em not be renumbered or reordered without incrementing
-// VERSION_MAJOR in ModuleFormat.h. Names, however, may change.
+// the version in ModuleFormat.h. Names, however, may change.
 FIRST_TYPE(BUILTIN_ALIAS, 1)
 TYPE(GENERIC_TYPE_PARAM)
 TYPE(DEPENDENT_MEMBER)

Diff for: include/swift/Serialization/ModuleFile.h

@@ -548,6 +548,11 @@ class ModuleFile
   /// 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);