ABI-checker: include Swift symbols only when ABI checking is enabled

ABI checking logics are largely Swift-specific, so we should exclude
symbols from the underlying Clang module.

rdar://64373983

tools/swift-api-digester/ModuleAnalyzerNodes.cpp

@@ -1573,6 +1573,7 @@ SwiftDeclCollector::constructInitNode(ConstructorDecl *CD) {
 bool swift::ide::api::
 SDKContext::shouldIgnore(Decl *D, const Decl* Parent) const {
   // Exclude all clang nodes if we're comparing Swift decls specifically.
+  // FIXME: isFromClang also excludes Swift decls with @objc. We should allow those.
   if (Opts.SwiftOnly && isFromClang(D)) {
     return true;
   }