Fix UB introduced by be0aa7b.

Author: qiongsiwu

clang/lib/Tooling/DependencyScanning/DependencyScannerImpl.cpp

@@ -714,12 +714,12 @@ bool CompilerInstanceWithContext::initialize(DiagnosticConsumer *DC) {
     DiagConsumer = &DiagPrinterWithOS->DiagPrinter;
   }
 
-  DiagEngineWithCmdAndOpts = std::make_unique<DignosticsEngineWithDiagOpts>(
-      CommandLine, OverlayFS, *DiagConsumer);
-
   std::tie(OverlayFS, CommandLine) = initVFSForByNameScanning(
       Worker.BaseFS, CommandLine, CWD, "ScanningByName");
 
+  DiagEngineWithCmdAndOpts = std::make_unique<DignosticsEngineWithDiagOpts>(
+      CommandLine, OverlayFS, *DiagConsumer);
+
   std::tie(Driver, Compilation) = buildCompilation(
       CommandLine, *DiagEngineWithCmdAndOpts->DiagEngine, OverlayFS, Alloc);