Add well-known features to the OSX CC toolchain to support configurin…

[paulherman]

I've not opened a feature request in bazelbuild/bazel but in bazelbuild/rules_cc as I'm not yet clear around the distinction between the two (bazelbuild/rules_cc#139).

I plan to follow-up with changes to the other toolchains and afterwards to docs as separate PRs.

One thing to note is that not all actions have C counterparts. There is another issue opened (#10411) for that, but tests all pass as things are today.

[oquenchil]

@googlewalt could you please take a look?

[googlewalt]

@keith Can you take a look to see if this is reasonable?

[googlewalt]

IIUC this implementation requires that user disable std_cxx11 in order to enable a diffferent C++ standard, which seems a little hard to use. Is that what we want?

Another concern is this will hard fail if multiple langauge standard features are specified. This is good for error checking. I think this is probably ok, but I'm not sure if there may be cases where hard fail is undesirable.

A couple other possible ways to implement this:

• Don't enable any of these features by default. Instead, set up a flag_group that adds the default standard if no language feature is selected. This would be my vote.

• Don't set the "provides" clause in the features. Order the default feature first, and have it enabled true. This way other features will override the default. This implementation would also not fail if multiple standard features for the same language are specified.

[brentleyjones]

Will need to rebase at a minimum, because this toolchain doesn't exist in Bazel anymore, it's now over in https://github.com/bazelbuild/apple_support/tree/master/crosstool. Opening an issue/PR over there, and/or discussing it in the #rules_apple-maintainers-discuss Slack channel would be my recommendation.