Remove \brief commands from doxygen comments.

We've been running doxygen with the autobrief option for a couple of
years now. This makes the \brief markers into our comments
redundant. Since they are a visual distraction and we don't want to
encourage more \brief markers in new code either, this patch removes
them all.

Patch produced by

      for i in $(git grep -l '\\brief'); do perl -pi -e 's/\\brief //g' $i & done

include/swift/Serialization/ModuleFile.h

@@ -703,7 +703,7 @@ public:
   /// Note that this may cause other decls to load as well.
   void loadExtensions(NominalTypeDecl *nominal);
 
-  /// \brief Load the methods within the given class that produce
+  /// Load the methods within the given class that produce
   /// Objective-C class or instance methods with the given selector.
   ///
   /// \param classDecl The class in which we are searching for @objc methods.