Create fallback event handler library for Swift Testing and XCTest interop

[jerryjrchen]

Minimal version of the changes in Jonathan's original PR here: #1280

The new library is renamed to be "_TestingInterop" to more closely match its intended (and limited) purpose, but that's open to further discussion.

Motivation:

This library will eventually be visible in the toolchain, and either testing library (Swift Testing or XCTest) will be able to use it to pass along unhandled issues to a test runner from the other framework, enabling interoperability.

Modifications:

Fallback handler is currently unchanged from #1280. The docs have been updated to describe intended usage.

No tests are included in this change because the interop library must make it into the toolchain first before we can write tests against it. Tests will be coming in the near future!

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.
• Toolchain build succeeds
• Expected symbols are visible in the toolchain build*

Ubuntu:

~/Downloads/usr ❯ nm lib/swift/linux/lib_TestingInterop.so | rg handler
• <stdin>
1:0000000000002140 b $s15_TestingInterop21_fallbackEventHandler33_AF82E98923290C49994CD1CA5A7BD647LL15Synchronization6AtomicVySVSgGvp
2:0000000000000b50 T $s15_TestingInterop38_swift_testing_getFallbackEventHandlerySPys4Int8VG_SVSiSVSgtYbXCSgyF
3:0000000000000b70 T $s15_TestingInterop42_swift_testing_installFallbackEventHandlerySbySPys4Int8VG_SVSiSVSgtYbXCF
57:0000000000000b40 T _swift_testing_getFallbackEventHandler
58:0000000000000b60 T _swift_testing_installFallbackEventHandler

macOS:

~/Downloads/Library ❯ nm Developer/Toolchains/swift-PR-84971-2110.xctoolchain/usr/lib/swift/macosx/testing/lib_TestingInterop.dylib | rg handler
• <stdin>
1:0000000000003bbc t _$s15_TestingInterop21_fallbackEventHandler33_AF82E98923290C49994CD1CA5A7BD647LL_WZ
2:0000000000008010 b _$s15_TestingInterop21_fallbackEventHandler33_AF82E98923290C49994CD1CA5A7BD647LL_Wz
3:0000000000008018 b _$s15_TestingInterop21_fallbackEventHandler33_AF82E98923290C49994CD1CA5A7BD647LLs13ManagedBufferCyySPys4Int8VG_SVSiSVSgtYbXCSgSo16os_unfair_lock_sVGvp
4:0000000000003c50 T _$s15_TestingInterop38_swift_testing_getFallbackEventHandlerySPys4Int8VG_SVSiSVSgtYbXCSgyF
5:0000000000003d24 T _$s15_TestingInterop42_swift_testing_installFallbackEventHandlerySbySPys4Int8VG_SVSiSVSgtYbXCF
46:0000000000003bf0 T __swift_testing_getFallbackEventHandler
47:0000000000003cb0 T __swift_testing_installFallbackEventHandler

Windows does not have the interop lib yet since it requires changes to https://github.com/swiftlang/swift-installer-scripts. This will be done in a follow-up :)

[grynspan]

Please hold off merging until the toolchain builds have finished and you've confirmed the binaries are emitted with the right exported symbols. Thanks!