DRTVWR-558: Generalize LLEventDispatcher::add() constraints.

Instead of checking whether an add() parameter is exactly LLSD or LLSDMap,
check whether it's convertible to LLSD -- which handles those cases and more.

(cherry picked from commit fa168c11f64771dadc5df86d14ca2f07eba3b8ba)

indra/llcommon/lleventdispatcher.h

@@ -192,8 +192,7 @@ class LL_COMMON_API LLEventDispatcher
     template<typename Method, typename InstanceGetter>
     typename std::enable_if<
         boost::function_types::is_member_function_pointer<Method>::value &&
-        ! std::is_same<InstanceGetter, LLSD>::value &&
-        ! std::is_same<InstanceGetter, LLSDMap>::value
+        ! std::is_convertible<InstanceGetter, LLSD>::value
         >::type add(const std::string& name,
                     const std::string& desc,
                     Method f,
@@ -247,8 +246,7 @@ class LL_COMMON_API LLEventDispatcher
     template<typename Method, typename InstanceGetter>
     typename std::enable_if<
         boost::function_types::is_member_function_pointer<Method>::value &&
-        ! std::is_same<InstanceGetter, LLSD>::value &&
-        ! std::is_same<InstanceGetter, LLSDMap>::value
+        ! std::is_convertible<InstanceGetter, LLSD>::value
         >::type add(const std::string& name,
                     const std::string& desc,
                     Method f,
@@ -492,8 +490,7 @@ LLEventDispatcher::add(const std::string& name, const std::string& desc, Functio
 template<typename Method, typename InstanceGetter>
 typename std::enable_if<
     boost::function_types::is_member_function_pointer<Method>::value &&
-    ! std::is_same<InstanceGetter, LLSD>::value &&
-    ! std::is_same<InstanceGetter, LLSDMap>::value
+    ! std::is_convertible<InstanceGetter, LLSD>::value
 >::type
 LLEventDispatcher::add(const std::string& name, const std::string& desc, Method f,
                        const InstanceGetter& getter)
@@ -516,8 +513,7 @@ LLEventDispatcher::add(const std::string& name, const std::string& desc, Functio
 template<typename Method, typename InstanceGetter>
 typename std::enable_if<
     boost::function_types::is_member_function_pointer<Method>::value &&
-    ! std::is_same<InstanceGetter, LLSD>::value &&
-    ! std::is_same<InstanceGetter, LLSDMap>::value
+    ! std::is_convertible<InstanceGetter, LLSD>::value
 >::type
 LLEventDispatcher::add(const std::string& name, const std::string& desc, Method f,
                        const InstanceGetter& getter,