The `IsABIPlaceholder` field was originally introduced over 5 years ago with
https://github.com/swiftlang/swift/pull/23731. It does not seem like anything
actually depends on this information, though, and the computation of the value
has become out-of-date as visionOS and anyAppleOS availability have been
introduced. Remove the field from the API digester JSON to avoid confusing
anybody else.
The framework baselines are installed at 'lib/swift/FrameworkABIBaseline' and the
tool is inside 'bin'. This patch teaches the executable to locate baselines
from the relative path.
In addition, this patch moves the stdlib ABI/API baselines to the canonical
location so we don't have to check the stability of the stdlib using a
different mechanism from other Swift frameworks.
Allan Shortlidge
Hamish Knight
Anthony Latsis
Xi Ge
eeckstein