Xcode-generated headers of the Swift standard library — updated with every beta.
To compare the stdlib headers between 2 betas, go to:
https://github.com/radex/swift_stdlib/compare/6.1-beta1...6.1-beta2.diff
And replace 6.1-beta1 and 6.1-beta2 with the two versions you're interested in. Versions are tagged like so: 6.0-beta2, 6.0-gm, 6.1-gm-seed2, 6.0.1. To see all the version tags, see https://github.com/radex/swift_stdlib/releases.
Note that at the moment, the order of function overrides appears quite random (rdar://18202155) — that's why in every beta there's a lot of lines changed, but there are usually few actual changes hidden in the noise.
btw. Swifter is a handy reference which provides auto-generated documentation for the Swift stdlib, with the ability to switch between versions.