Add support for Android

[grynspan]

Add support for Android:

• Audit our platform-specific code and add branches for Android (mostly, but not entirely, the same as Linux)
• Set up team workstations or VMs
• Set up CI jobs

[finagolfin]

Note that while cross-compiling some proportion of Testing tests has worked for a while now, long-standing SwiftPM bugs like swiftlang/swift-package-manager#8094 have broken cross-compiling this repo's tests to any platform, so the remaining steps here are blocked on those underlying bugs, which I'm told the new swift-build backend may soon fix. Instead, I have been occasionally natively building the trunk toolchain on Android in the Termux app then testing this repo, and with #1373 or an earlier patch, all tests passed.

[grynspan]

@finagolfin @compnerd Any thoughts on how we can run our test suite automatically on Android as part of our GHA workflow suite? If we can do that and we can sort out instructions for at-desk testing, we can mark Android as fully supported I believe.

[compnerd]

Yes, that is possible; https://github.com/compnerd/ds2 was running the tests on Android, so it is definitely possible to do in GHA.

[finagolfin]

There are potentially two separate issues with cross-compiling Testing tests for Android:

1. We need to check if the recent swift-build switch in trunk SwiftPM fixed the problems with using macros, or if more work is needed in SwiftPM.
2. Once a test runner is successfully produced, it should just work with the current official workflows, which @marcprux already has working with Testing, provided packages don't define their own macros and avoid that long-standing SwiftPM bug.

We won't know if 2. works for sure till we fix 1., but the emulator testing probably works fine already.

[marcprux]

We still can't cross-compile the Testing tests due to swiftlang/swift-package-manager#8094. I just tried again with 6.3.1 and got the expected failure:

zap swift-android-sdk/swift-testing [android-checks] % ANDROID_SERIAL=emulator-5554 skip android test

[0/1] Planning build
Building for debugging...
[0/21] Write sources
[37/65] Write swift-version-27A8E6E6C94AF7AD.txt
[38/65] Write sources
[39/67] Compiling _TestingInternals Discovery.cpp
…
[784/791] Compiling TestingTests EventHandlingInteropTests.swift
[785/791] Emitting module TestingTests
error: Failed opening '/opt/src/github/swift-android-sdk/swift-testing/.build/aarch64-unknown-linux-android28/debug/index/store/v5/units/ABI.EncodedTestTests.swift.o-IOQ08LTW0GJI': No such file or directory
[785/791] /opt/src/github/swift-android-sdk/swift-testing/.build/aarch64-unknown-linux-android28/debug/swift-testingPackageDiscoveredTests.derived/all-discovered-tests.swift
Error: Error terminated(code: 1) running command: /Users/marc/Library/Developer/Toolchains/swift-6.3.1-RELEASE.xctoolchain/usr/bin/swift build --swift-sdk aarch64-unknown-linux-android28 --build-tests -Xswiftc -plugin-path -Xswiftc /Users/marc/Library/Developer/Toolchains/swift-6.3.1-RELEASE.xctoolchain/usr/lib/swift/host/plugins/testing -Xswiftc -DSKIP_BRIDGE -Xswiftc -DTARGET_OS_ANDROID

For GHA CI, you can activate Android testing by just turning on enable_android_sdk_checks: true (in addition to enable_android_sdk_build: true, which is already enabled), but as expected, it also fails in my attempt: swift-android-sdk#1

To be clear, tests that use Testing do work fine on Android (provided the package under test does not define its own additional macros), but Testing itself cannot be tested in Android due to this issue. I hope it gets fixed someday!