[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)
  }
  }