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.
