Add StructuredData plugin type; showcase with new DarwinLog feature

Take 2, with missing cmake line fixed.  Build tested on
Ubuntu 14.04 with clang-3.6.

See docs/structured_data/StructuredDataPlugins.md for details.

differential review: https://reviews.llvm.org/D22976

reviewers: clayborg, jingham
llvm-svn: 279202
diff --git a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteClientBase.h b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteClientBase.h
index 9cfba4b..684ef3e 100644
--- a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteClientBase.h
+++ b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteClientBase.h
@@ -31,6 +31,16 @@
         HandleAsyncMisc(llvm::StringRef data) = 0;
         virtual void
         HandleStopReply() = 0;
+
+        //
+        /// Processes async structured data.
+        ///
+        /// @return
+        ///    true if the data was handled; otherwise, false.
+        //
+        virtual bool
+        HandleAsyncStructuredData(const StructuredData::ObjectSP
+                                  &object_sp) = 0;
     };
 
     GDBRemoteClientBase(const char *comm_name, const char *listener_name);