[donotmerge] span implementation
Slightly further along than https://github.com/swiftlang/swift/pull/76406; building a new toolchain here in order to avoid blowing out the artifacts in the other PR.
@swift-ci please build toolchain macOS
@swift-ci please build toolchain macOS
@swift-ci please smoke test macOS platform
@swift-ci please build toolchain macOS
@swift-ci please build toolchain
@swift-ci please smoke test
@swift-ci please build toolchain windows platform
@swift-ci please smoke test linux platform
@swift-ci please smoke test linux platform
@swift-ci please smoke test linux platform
@swift-ci please smoke test macOS platform