Metal: Refactor textureGrad wrappers * Removed variadic macros. * Removed an invalid overload. * Updated emitter to not output unused variants. * Added MSL compile guards to not fail shader compilation on macOS 10.15. Bug: angleproject:368059227 Change-Id: I816b8b96b264d0ca943bca566987755460b3a72f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5893778 Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>