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/SILOptimizer/LoopTransforms/COWArrayOpt.cpp

@@ -379,7 +379,7 @@ class COWArrayOpt {
   // Set of all blocks that may reach the loop, not including loop blocks.
   llvm::SmallPtrSet<SILBasicBlock*,32> ReachingBlocks;
 
-  /// \brief Transient per-Array user set.
+  /// Transient per-Array user set.
   ///
   /// Track all known array users with the exception of struct_extract users
   /// (checkSafeArrayElementUse prohibits struct_extract users from mutating the