A number of tests exercise features only available in Apple OSes that
shipped with Swift 5.0 in the OS; this includes the following versions:
- macOS 10.14.4
- iOS 12.2
- tvOS 12.2
- watchOS 5.2
Previously these tests were restricted to running on macOS only, with
an explicit -target x86_64-apple-macosx10.14.4. To get better test
coverage, add a new %target-stable-abi-triple substitution which
expands to a triple with the correct OS version on all Apple platforms.
On non-Apple platforms, this is the same as %target-variant-triple,
but for now any test that uses this exercises Apple platform features
anyway.
One caveat is that since iOS 12.2 does not have a 32-bit slice, we
have to skip any tests that use -target %target-stable-abi-triple
on this platform. A new swift_stable_abi feature flag can be tested
with 'REQUIRES: swift_stable_abi'. To get maximum test coverage,
I split off a 'stable_abi' version of a few tests that build with both
an old and new deployment target. This allows the old deployment
target case to still be tested on 32-bit iOS.
