Upstream Apple's direct-to-Metal backend: compile translator. This change is meant to merge the translator changes from Apple's direct-to-Metal backend. Taken from Kyle Piddington's CL: https://chromium-review.googlesource.com/c/angle/angle/+/2857366/ The goal of this CL is to merge the translator code in a state that compiles, but not to switch the Metal backend over to use this translator backend yet. Bug: angleproject:5505 Change-Id: I68a6354604498cd5fd1eb96c13fc56f3b38f2bd0 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2897536 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>