Most of this is enabled by an upstream change to generalize the tests
for the api-digester to properly handle all targets. The primary change
here is to account for the Swift-level ABI differences for the standard
library between Apple Silicon macOS and Intel macOS. Specifically, anything
related to Float80 will be removed from the standard library on Apple Silicon
macOS, so account for the differences.
With module stability from the compiler, we can now use an earlier version
of the SDK as the baseline and compare the new SDK with it. This patch
adds a new -bsdk flag to the tool to specify the baseline SDK path.