swift-api-checker.py: add -swift-only flag when checking ABI stability

The existing logic of ABI checking only works for Swift symbols. We need to
add this flag so the script can work with Swift overlays.

rdar://60786484

Author: nkcsgexi

utils/api_checker/swift-api-checker.py

@@ -151,7 +151,7 @@ def run(self, output, module, swift_ver, opts,
         for path in self.inputs:
             cmd.extend(['-I', path])
         if self.abi:
-            cmd.extend(['-abi'])
+            cmd.extend(['-abi', '-swift-only'])
         cmd.extend(['-' + o for o in opts])
         if self.verbose:
             cmd.extend(['-v'])