Vulkan: Use SPIR-V ids instead of names in the transformer This change removes the SPIR-V transformer's reliance on type and variable names. As a result: - String hashing is removed from the info map data structure and the SPIR-V transformer - The ID discovery class is entirely removed - Internal variable names have become a detail of the compiler alone (and are no longer exposed as part of the compiler interface) - Some front-end name tracking is removed ("parentStructMappedName", etc) This change also properly cleans up xfb emulation types that were previously left over. This change allows the SPIR-V compiler to emit user strings as-is instead of prefixing them with `u_` leading to more readable debug shaders. Additionally, it will make it possible not to emit debug info at all. Both of these changes will be done in follow ups. Bug: angleproject:7220 Change-Id: Iaa127496209a27aaae2e0d14c41b22fffb0b72a2 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4600610 Reviewed-by: Roman Lavrov <romanl@google.com> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>