Metal: Refactor textureLod 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: If15fcb0cb772e3ee2e2e1669f894fb8de4e76578 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5896390 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>