Vulkan: Rename DescriptorSetLayoutDesc update() to addBinding() The `update()` method is never actually used to update the exiting bindings (but rather to add new ones), this change renames the method to `addBinding()` and adds few ASSERTs for clarity. Also, after recent changes in `DescriptorSetLayoutDesc` class, some changes made by `update()` method are irreversible. It is possible to have different descriptions that will produce same layout if use `update()` to rewrite the existing structure. Bug: angleproject:8677 Change-Id: If85eb2b271bc06843ee9326c024d73801d3da091 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5676345 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>