Use the proper `swift_only_stable_abi` check to ensure that the tests do
not run on targets that do not support the pre-stable ABI.
Thanks to @slavapestov for pointing out that there was a better way!
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.
If a class does not have a custom @objc name, objc_getClass() can find
it at runtime by calling the Swift runtime's metadata demangler hook.
This avoids the static initializer on startup. If the class has a
custom runtime name we still need the static initializer unfortunately.
Fixes <rdar://problem/49660515>.
