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/Basic/JSONSerialization.h

@@ -11,7 +11,7 @@
 //===----------------------------------------------------------------------===//
 //
 /// \file
-/// \brief Provides an interface for serializing to JSON.
+/// Provides an interface for serializing to JSON.
 /// \note This does not include support for deserializing JSON; since JSON is
 /// a subset of YAML, use LLVM's YAML parsing support instead.
 ///