Fix default and driver uniforms' binding indices. These abstractions were introduced in the SymbolTable during upstreaming of the direct-to-Metal compiler and were accidentally swapped, leading to widespread test failures. Fix the comment reference in ContextMtl's DriverUniforms struct. Add missing emulatedInstance field, and add a TODO about its missing support in direct-to-Metal translator. Define it in SPIR-V Metal translator; the driver uniforms for the two must match. Bug: angleproject:6259 Change-Id: I0330ab4efca5b64a749b9cca90564fdbc84d7a80 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3078998 Commit-Queue: Kenneth Russell <kbr@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>