[SR-11795] Replace OSX with macOS pretty much everywhere

Previous ID	SR-11795
Radar	rdar://35127522
Original Reporter	@CodaFi
Type	Bug
Status	In Progress
Resolution

Votes	1
Component/s	Compiler
Labels	Bug, StarterBug
Assignee	Tudes100 (JIRA)
Priority	Medium

md5: 01b356d7a526d3a65852312ab839b033

Issue Description:

The codebase still has quite a few places where we use the spelling OSX instead of macOS, especially in API availability stuff. While not all usages of OS X are incorrect - in particular, it is not correct to simply find-and-replace all the usages of "OS X" and "OSX" with "macOS", it's still probably a good idea to scrub the codebase of the non-essential ones. Graydon has already done most of the heavy lifting, so the end result is just responding to the feedback on https://github.com/apple/swift/pull/12674, rebasing it, and running the tests to verify the changes.