allow swift-symbolgraph-extract to be called on non-swift modules (#42399)

rdar://90842354

Author: QuietMisdreavus

Diff for: lib/DriverTool/swift_symbolgraph_extract_main.cpp

@@ -223,7 +223,10 @@ int swift_symbolgraph_extract_main(ArrayRef<const char *> Args,
     return EXIT_FAILURE;
   }
 
-  const auto &MainFile = M->getMainFile(FileUnitKind::SerializedAST);
+  FileUnitKind expectedKind = FileUnitKind::SerializedAST;
+  if (M->isNonSwiftModule())
+    expectedKind = FileUnitKind::ClangModule;
+  const auto &MainFile = M->getMainFile(expectedKind);
 
   if (Options.PrintMessages)
     llvm::errs() << "Emitting symbol graph for module file: " << MainFile.getModuleDefiningPath() << '\n';