iOS: Treat warnings as errors

[hramos]

Summary

Related to #22609. Prevent new warnings from being introduced into the codebase by treating them as errors in CI.

Changelog

[iOS] [Changed] - Xcode warnings are treated as errors during CI tests.

Test Plan

See https://circleci.com/workflow-run/147742c3-69ee-439c-8cc4-6dd0cbc8726f for an example of a failed build, as there's a couple of warnings that exist on master as of this writing:

react-native/ReactCommon/jsi/JSCRuntime.cpp:827:65: error: unused parameter 'obj' [-Werror,-Wunused-parameter]
jsi::WeakObject JSCRuntime::createWeakObject(const jsi::Object& obj) {
                                                                ^
react-native/ReactCommon/jsi/JSCRuntime.cpp:837:62: error: unused parameter 'obj' [-Werror,-Wunused-parameter]
jsi::Value JSCRuntime::lockWeakObject(const jsi::WeakObject& obj) {
                                                             ^
2 errors generated.

[cpojer]

Nice, this is super awesome! Land it if it passes CI.

[hramos]

I am not sure yet if this is the right approach. I fixed the warnings in the main project, but clang is now failing on warnings from third-party dependencies. E.g. folly:

react-native/third-party/folly-2018.10.22.00/folly/detail/Demangle.cpp:33:23: error: unused parameter 'mangled' [-Werror,-Wunused-parameter]
    char const* const mangled,
                      ^
react-native/third-party/folly-2018.10.22.00/folly/detail/Demangle.cpp:34:18: error: unused parameter 'cbref' [-Werror,-Wunused-parameter]
    void (*const cbref)(char const*, std::size_t, void*),
                 ^
react-native/third-party/folly-2018.10.22.00/folly/detail/Demangle.cpp:35:17: error: unused parameter 'opaque' [-Werror,-Wunused-parameter]
    void* const opaque) {
                ^
3 errors generated.

[cpojer]

Aww that sucks. Maybe another approach could simply be to build with warnings and fail it if the output (stderr/stdout) text matches warnings in RN?