[DependencyScanning] Make 'ModuleInfo.ImportInfos' 'nil' when using a toolchain which does not support querying them.

Author: artemcm

Sources/SwiftDriver/SwiftScan/DependencyGraphBuilder.swift

@@ -104,19 +104,19 @@ private extension SwiftScan {
       }
     }
 
-    var importInfos: [ImportInfo] = []
+    var importInfos: [ImportInfo]? = nil
     if supportsImportInfos {
       let importInfoSetRefOrNull = api.swiftscan_module_info_get_imports(moduleInfoRef)
       guard let importInfoSetRef = importInfoSetRefOrNull else {
         throw DependencyScanningError.missingField("dependency_graph.imports")
       }
       let importInfoRefArray = Array(UnsafeBufferPointer(start: importInfoSetRef.pointee.imports,
                                                          count: Int(importInfoSetRef.pointee.count)))
-      for importInfoRefOrNull in importInfoRefArray {
+      importInfos = try importInfoRefArray.map { importInfoRefOrNull in
         guard let importInfoRef = importInfoRefOrNull else {
           throw DependencyScanningError.missingField("dependency_set_t.imports[_]")
         }
-        importInfos.append(try constructImportInfo(from: importInfoRef))
+        return try constructImportInfo(from: importInfoRef)
       }
     }