SL-16202: Instantiate LLSimpleton::sInstance generically

instead of requiring a separate declaration for each subclass.

The previous way produces errors in clang.

indra/llcommon/llsingleton.h

@@ -865,4 +865,7 @@ public:
     }
 };
 
+template <class T>
+T* LLSimpleton<T>::sInstance{ nullptr };
+
 #endif

indra/llui/llviewereventrecorder.cpp

@@ -28,8 +28,6 @@
 #include "llui.h"
 #include "llleap.h"
 
-LLViewerEventRecorder* LLSimpleton<LLViewerEventRecorder>::sInstance = nullptr;
-
 LLViewerEventRecorder::LLViewerEventRecorder() {
 
   clear(UNDEFINED);

indra/newview/llenvironment.cpp

@@ -824,7 +824,6 @@ std::string env_selection_to_string(LLEnvironment::EnvSelection_t sel)
 #undef RTNENUM
 }
 
-LLEnvironment* LLSimpleton<LLEnvironment>::sInstance = nullptr;
 //-------------------------------------------------------------------------
 LLEnvironment::LLEnvironment():
     mCloudScrollDelta(),

indra/newview/llselectmgr.cpp

@@ -97,8 +97,6 @@
 #include "llglheaders.h"
 #include "llinventoryobserver.h"
 
-LLSelectMgr* LLSimpleton<LLSelectMgr>::sInstance = nullptr;
-
 LLViewerObject* getSelectedParentObject(LLViewerObject *object) ;
 //
 // Consts

indra/newview/llviewercamera.cpp

@@ -54,8 +54,6 @@
 // System includes
 #include <iomanip> // for setprecision
 
-LLViewerCamera* LLSimpleton<LLViewerCamera>::sInstance = nullptr;
-
 LLTrace::CountStatHandle<> LLViewerCamera::sVelocityStat("camera_velocity");
 LLTrace::CountStatHandle<> LLViewerCamera::sAngularVelocityStat("camera_angular_velocity");
 

indra/newview/llworld.cpp

@@ -62,8 +62,6 @@
 #include <cstring>
 
 
-LLWorld* LLSimpleton<LLWorld>::sInstance = nullptr;
-
 //
 // Globals
 //