It's main advantage over vkd3d_shader_message_context_copy_messages() is
that it can't fail. The original issue this addresses is that
vkd3d_shader_compile() should free its output when
vkd3d_shader_message_context_copy_messages() fails, as spotted by
Giovanni; that likely would have applied to a number of the other uses
of vkd3d_shader_message_context_copy_messages() as well.