Rename instances of “OS X” to “macOS”

[tonyarnold]

This is a reworking of #12674, as rebasing became a bit of a dead end.

Here is @graydon's original PR description:

It's been pointed out that swiftc still uses 'OSX' or 'OS X' in a lot of places it ought to be saying 'macOS' now. This change switches many, but not all, of those cases. In particular it fixes those cases where the compiler gives feedback about availability guards (the original report was about these) as well as switching internal symbols and docs.

Notably: this change does not eliminate support for 'OSX' in availability checks (it's still a synonym) nor does it change all of the tests that use 'OSX' in those checks; I figured there was no point since the synonym is likely to be supported in perpetuity and it's good to test that fact too. Moreover this change does not alter the terminology in any of the configury or build system; it's my hope that by avoiding that, I can avoid unintentional breakage, though I suspect I'll still be breaking something.

Opinions welcome on how else to stage this.

rdar://35127522

Resolves #54205.

[Rish-it]

i don't have a permission to commit
and upstream is already exist

[tonyarnold]

@Rish-it you'll need to fork the repository to your own account, and push a new branch there. You won't be able to push to Graydon's original fork and branch, nor to the main Swift repository.

See the GettingStarted.md documentation for more details: https://github.com/apple/swift/blob/main/docs/HowToGuides/GettingStarted.md#setting-up-your-fork