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

lib/Sema/CodeSynthesis.cpp

@@ -2186,7 +2186,7 @@ void TypeChecker::synthesizeFunctionBody(SynthesizedFunction fn) {
   llvm_unreachable("bad synthesized function kind");
 }
 
-/// \brief Create an implicit struct or class constructor.
+/// Create an implicit struct or class constructor.
 ///
 /// \param decl The struct or class for which a constructor will be created.
 /// \param ICK The kind of implicit constructor to create.