[Concurrency] Rename Job to ExecutorJob, ease transition with typealias

[ktoso]

According to SE review, we want to not take over the Job name.

This attempts to first introduce a typealias, and then we could maybe remove Job entirely.

Looking for input if this strategy makes sense.

[ktoso]

@swift-ci please smoke test

[ktoso]

preset=stdlib_S_standalone_minimal_macho_x86_64,build,test
@swift-ci please clean test with toolchain and preset

[ktoso]

ExecutorJob/Job need to be handled manually in the "only one of overloads can be implemented" because of how we provide default impls for both. Glad that the tests caught that nicely.

Added the special handling in tryDiagnoseExecutorConformance if we get to remove Job we can undo the last commit from here

[ktoso]

@swift-ci please smoke test

[ktoso]

@swift-ci please clean smoke test macos

[ktoso]

Renaming job sadly overlapped with adding a test which used job type in message... re-running with adjustment.

[ktoso]

@swift-ci please smoke test

[ktoso]

Our good ol' random failure friend:

macos.swiftinterface:4:8: error: no such module 'Combine'
import Combine
^

[ktoso]

@swift-ci please smoke test macos