Fix ResourceManager-related linker errors on Clang.

Guess this will break something else.

src/Magnum/DebugTools/ResourceManager.cpp

@@ -37,7 +37,7 @@
 namespace Magnum {
 
 namespace Implementation {
-    template MAGNUM_DEBUGTOOLS_EXPORT ResourceManager<Implementation::ResourceManagerLocalInstance, AbstractShaderProgram, Buffer, Mesh, MeshView, DebugTools::ForceRendererOptions, DebugTools::ObjectRendererOptions, DebugTools::ShapeRendererOptions>*& ResourceManagerLocalInstanceImplementation<ResourceManagerLocalInstance, AbstractShaderProgram, Buffer, Mesh, MeshView, DebugTools::ForceRendererOptions, DebugTools::ObjectRendererOptions, DebugTools::ShapeRendererOptions>::internalInstance();
+    template struct MAGNUM_DEBUGTOOLS_EXPORT ResourceManagerLocalInstanceImplementation<ResourceManagerLocalInstance, AbstractShaderProgram, Buffer, Mesh, MeshView, DebugTools::ForceRendererOptions, DebugTools::ObjectRendererOptions, DebugTools::ShapeRendererOptions>;
 }
 
 namespace DebugTools {

src/Magnum/Test/ResourceManagerLocalInstanceTestLib.cpp

@@ -30,7 +30,7 @@
 namespace Magnum {
 
 namespace Implementation {
-    template MAGNUM_RESOURCEMANAGERLOCALINSTANCETESTLIB_EXPORT ResourceManager<ResourceManagerLocalInstance, Int>*& ResourceManagerLocalInstanceImplementation<ResourceManagerLocalInstance, Int>::internalInstance();
+    template struct CORRADE_VISIBILITY_EXPORT ResourceManagerLocalInstanceImplementation<ResourceManagerLocalInstance, Int>;
 }
 
 namespace Test {