Bazel warnings for iOS libraries

Defines a bazel objc_library target with strict Objective-C warnings: strict_warnings_objc_library.

Add the following to your WORKSPACE:

git_repository(
    name = "bazel_ios_warnings",
    remote = "https://github.com/material-foundation/bazel_ios_warnings.git",
    tag = "v3.0.0",
)

In your BUILD file, use strict_warnings_objc_library just like you'd use objc_library:

load("@bazel_ios_warnings//:strict_warnings_objc_library.bzl", "strict_warnings_objc_library")

strict_warnings_objc_library(
)