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/IDE/CodeCompletion.cpp

@@ -1339,7 +1339,7 @@ class CodeCompletionCallbacksImpl : public CodeCompletionCallbacks {
     Builder.addTypeAnnotation(ST.getString());
   }
 
-  /// \brief Set to true when we have delivered code completion results
+  /// Set to true when we have delivered code completion results
   /// to the \c Consumer.
   bool DeliveredResults = false;
 
@@ -1636,10 +1636,10 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
 
   bool IncludeInstanceMembers = false;
 
-  /// \brief True if we are code completing inside a static method.
+  /// True if we are code completing inside a static method.
   bool InsideStaticMethod = false;
 
-  /// \brief Innermost method that the code completion point is in.
+  /// Innermost method that the code completion point is in.
   const AbstractFunctionDecl *CurrentMethod = nullptr;
 
   Optional<SemanticContextKind> ForcedSemanticContext = None;