The presence of Copyable/Escapable conformances doesn't affect ABI. Only
their absence in terms of suppressed requirements like `~Copyable` need
to be output for diffing by the APIDigester.
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.
We used to use a Json format to capture the ABI/API detail of Swift modules to
walk-around module compatibility issues across different compiler versions. Since
Swift module now is in stable format, we can compare two swiftinterface files
directly without dumping the content into Json format.
