Fix mismatched class/struct definition warnings [-Wmismatched-tags]

Fixes the following Clang 7 warnings:
```
core/object.cpp:44:1: warning: '_ObjectDebugLock' defined as a struct here but previously declared as a class [-Wmismatched-tags]
core/variant_call.cpp:43:1: warning: '_VariantCall' defined as a struct here but previously declared as a class [-Wmismatched-tags]
drivers/gles3/rasterizer_storage_gles3.h:765:2: warning: 'MultiMesh' defined as a struct here but previously declared as a class [-Wmismatched-tags]
editor/editor_node.h:794:1: warning: 'EditorProgress' defined as a struct here but previously declared as a class [-Wmismatched-tags]
modules/bullet/rigid_body_bullet.h:230:17: warning: class 'KinematicUtilities' was previously declared as a struct [-Wmismatched-tags]
modules/bullet/space_bullet.h:60:1: warning: class 'btSoftBodyWorldInfo' was previously declared as a struct [-Wmismatched-tags]
scene/resources/world_2d.cpp:40:1: warning: 'SpatialIndexer2D' defined as a struct here but previously declared as a class [-Wmismatched-tags]
scene/resources/world.cpp:39:1: warning: 'SpatialIndexer' defined as a struct here but previously declared as a class [-Wmismatched-tags]
servers/audio/reverb_sw.cpp:60:1: warning: 'ReverbParamsSW' defined as a struct here but previously declared as a class [-Wmismatched-tags]
thirdparty/bullet/BulletSoftBody/btSoftBody.h:43:1: warning: 'btSoftBodyWorldInfo' defined as a struct here but previously declared as a class [-Wmismatched-tags]
```

core/object.h

@@ -422,7 +422,7 @@ private:
 	};
 
 #ifdef DEBUG_ENABLED
-	friend class _ObjectDebugLock;
+	friend struct _ObjectDebugLock;
 #endif
 	friend bool predelete_handler(Object *);
 	friend void postinitialize_handler(Object *);

core/variant.h

@@ -116,7 +116,7 @@ public:
 	};
 
 private:
-	friend class _VariantCall;
+	friend struct _VariantCall;
 	// Variant takes 20 bytes when real_t is float, and 36 if double
 	// it only allocates extra memory for aabb/matrix.
 

drivers/gles3/rasterizer_storage_gles3.h

@@ -691,7 +691,7 @@ public:
 		}
 	};
 
-	class MultiMesh;
+	struct MultiMesh;
 
 	struct Mesh : public GeometryOwner {
 

editor/editor_export.h

@@ -37,10 +37,10 @@
 #include "scene/main/timer.h"
 #include "scene/resources/texture.h"
 
-class EditorProgress;
 class FileAccess;
 class EditorExportPlatform;
 class EditorFileSystemDirectory;
+struct EditorProgress;
 
 class EditorExportPreset : public Reference {
 

modules/bullet/rigid_body_bullet.h

@@ -227,7 +227,7 @@ public:
 
 	void init_kinematic_utilities();
 	void destroy_kinematic_utilities();
-	_FORCE_INLINE_ class KinematicUtilities *get_kinematic_utilities() const { return kinematic_utilities; }
+	_FORCE_INLINE_ KinematicUtilities *get_kinematic_utilities() const { return kinematic_utilities; }
 
 	_FORCE_INLINE_ btRigidBody *get_bt_rigid_body() { return btBody; }
 

modules/bullet/space_bullet.h

@@ -57,7 +57,7 @@ class btDiscreteDynamicsWorld;
 class btEmptyShape;
 class btGhostPairCallback;
 class btSoftRigidDynamicsWorld;
-class btSoftBodyWorldInfo;
+struct btSoftBodyWorldInfo;
 class ConstraintBullet;
 class CollisionObjectBullet;
 class RigidBodyBullet;

scene/resources/world.h

@@ -36,9 +36,9 @@
 #include "servers/physics_server.h"
 #include "servers/visual_server.h"
 
-class SpatialIndexer;
 class Camera;
 class VisibilityNotifier;
+struct SpatialIndexer;
 
 class World : public Resource {
 	GDCLASS(World, Resource);

scene/resources/world_2d.cpp

@@ -29,13 +29,13 @@
 /*************************************************************************/
 
 #include "world_2d.h"
-#include "servers/physics_2d_server.h"
+
-#include "servers/visual_server.h"
-//#include "servers/spatial_sound_2d_server.h"
 #include "core/project_settings.h"
 #include "scene/2d/camera_2d.h"
 #include "scene/2d/visibility_notifier_2d.h"
 #include "scene/main/viewport.h"
+#include "servers/physics_2d_server.h"
+#include "servers/visual_server.h"
 
 struct SpatialIndexer2D {
 

scene/resources/world_2d.h

@@ -35,9 +35,9 @@
 #include "core/resource.h"
 #include "servers/physics_2d_server.h"
 
-class SpatialIndexer2D;
 class VisibilityNotifier2D;
 class Viewport;
+struct SpatialIndexer2D;
 
 class World2D : public Resource {
 

servers/audio/reverb_sw.cpp

@@ -29,33 +29,21 @@
 /*************************************************************************/
 
 #include "reverb_sw.h"
+
 #include "core/print_string.h"
-#include "stdlib.h"
+
+#include <stdlib.h>
+
 #define SETMIN(x, y) (x) = MIN((x), (y))
+
 #define rangeloop(c, min, max) \
 	for ((c) = (min); (c) < (max); (c)++)
 
 #define ABSDIFF(x, y) \
 	(((x) < (y)) ? ((y) - (x)) : ((x) - (y)))
 
-#ifdef bleh_MSC_VER
-
-#if _MSC_VER >= 1400
-_FORCE_INLINE_ int32_tMULSHIFT_S32(
-		int32_t Factor1,
-		int32_t Factor2,
-		uint8_t Bits) {
-
-	return __ll_rshift(
-			__emul(Factor1, Factor2),
-			Bits);
-}
-#endif
-
-#else
 #define MULSHIFT_S32(Factor1, Factor2, Bits) \
 	((int)(((int64_t)(Factor1) * (Factor2)) >> (Bits)))
-#endif
 
 struct ReverbParamsSW {
 	unsigned int BufferSize; // Required buffer size

servers/audio/reverb_sw.h

@@ -34,7 +34,7 @@
 #include "core/os/memory.h"
 #include "core/typedefs.h"
 
-class ReverbParamsSW;
+struct ReverbParamsSW;
 
 class ReverbSW {
 public: