[Dependency Scanning] Do not re-query a given clang module identifier more than once

During parallel clang module dependency resolution, an unintended side-effect of swiftlang#84929 is that we stopped uniquing the module identifiers we query to the Clang dependency scanner.

This change ensures we do not query the same identifier more than once.

Resolves rdar://165133617

Author: artemcm

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -1113,17 +1113,20 @@ void ModuleDependencyScanner::performParallelClangModuleLookup(
   };
 
   // Enque asynchronous lookup tasks
+  llvm::StringSet<> queriedIdentifiers;
   for (const auto &unresolvedImports : unresolvedImportsMap)
     for (const auto &unresolvedImportInfo : unresolvedImports.second)
-      ScanningThreadPool.async(
-          scanForClangModuleDependency,
-          getModuleImportIdentifier(unresolvedImportInfo.importIdentifier));
+      if (queriedIdentifiers.insert(unresolvedImportInfo.importIdentifier).second)
+        ScanningThreadPool.async(
+            scanForClangModuleDependency,
+            getModuleImportIdentifier(unresolvedImportInfo.importIdentifier));
 
   for (const auto &unresolvedImports : unresolvedOptionalImportsMap)
     for (const auto &unresolvedImportInfo : unresolvedImports.second)
-      ScanningThreadPool.async(
-          scanForClangModuleDependency,
-          getModuleImportIdentifier(unresolvedImportInfo.importIdentifier));
+      if (queriedIdentifiers.insert(unresolvedImportInfo.importIdentifier).second)
+        ScanningThreadPool.async(
+            scanForClangModuleDependency,
+            getModuleImportIdentifier(unresolvedImportInfo.importIdentifier));
 
   ScanningThreadPool.wait();
 }