ABIChecker: add an API to document that SDKNodeRoot contains single module name. NFC

Author: nkcsgexi

include/swift/APIDigester/ModuleAnalyzerNodes.h

@@ -446,6 +446,7 @@ class SDKNodeRoot: public SDKNode {
   ArrayRef<SDKNodeDecl*> getDescendantsByUsr(StringRef Usr) {
     return DescendantDeclTable[Usr].getArrayRef();
   }
+  llvm::Optional<StringRef> getSingleModuleName() const;
 };
 
 class SDKNodeType: public SDKNode {

lib/APIDigester/ModuleAnalyzerNodes.cpp

@@ -2107,6 +2107,13 @@ void SDKNodeRoot::jsonize(json::Output &out) {
     out.mapOptional(getKeyContent(Ctx, KeyKind::KK_tool_arguments).data(), ToolArgs);
 }
 
+llvm::Optional<StringRef> SDKNodeRoot::getSingleModuleName() const {
+  auto rawName = getName();
+  if (rawName == "MULTI_MODULE" || rawName == "NO_MODULE")
+    return llvm::None;
+  return rawName;
+}
+
 void SDKNodeConformance::jsonize(json::Output &out) {
   SDKNode::jsonize(out);
   output(out, KeyKind::KK_usr, Usr);