Cleanup: Add protection for void CPPType and GPointer

Previously using these types with a void type would give a
linker error. Compile errors are much easier to diagnose.

Pull Request: https://projects.blender.org/blender/blender/pulls/140296

source/blender/blenlib/BLI_cpp_type.hh

@@ -447,6 +447,9 @@ void register_cpp_types();
 
 namespace blender {
 
+/* Give a compile error instead of a link error when type information is missing. */
+template<> const CPPType &CPPType::get_impl<void>() = delete;
+
 /**
  * Two types only compare equal when their pointer is equal. No two instances of CPPType for the
  * same C++ type should be created.

source/blender/blenlib/BLI_generic_pointer.hh

@@ -100,7 +100,10 @@ class GPointer {
 
   GPointer(const CPPType &type, const void *data = nullptr) : type_(&type), data_(data) {}
 
-  template<typename T> GPointer(T *data) : GPointer(&CPPType::get<T>(), data) {}
+  template<typename T, BLI_ENABLE_IF((!std::is_void_v<T>))>
+  GPointer(T *data) : GPointer(&CPPType::get<T>(), data)
+  {
+  }
 
   operator bool() const
   {