[clang][deps] Make sure ScanInstance outlives collector

[jansvoboda11]

Summary

The ScanInstance is a local variable in DependencyScanningAction::runInvocation() that is referenced by ModuleDepCollector. Since D132405, ModuleDepCollector can escape the function and can outlive its ScanInstance. This patch fixes that.

[jansvoboda11]

Another option would be to store ScanInstance as member of DependencyScanningAction.

[jansvoboda11]

Replace std::shared_ptr by Optional stored on DependencyScanningAction.

[benlangmuir]

Good catch, thanks!

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

if (Scanned) {
       // FIXME: to support multi-arch builds, each arch requires a separate scan
       setLastCC1Arguments(std::move(OriginalInvocation));
       return true;
     }
 
     Scanned = true;
 
     // Create a compiler instance to handle the actual work.
-    CompilerInstance ScanInstance(std::move(PCHContainerOps));
+    ScanInstanceStorage.emplace(std::move(PCHContainerOps));
+    CompilerInstance &ScanInstance = *ScanInstanceStorage;
     ScanInstance.setInvocation(std::move(Invocation));
 
     // Create the compiler's actual diagnostics engine.
     sanitizeDiagOpts(ScanInstance.getDiagnosticOpts());
     ScanInstance.createDiagnostics(DiagConsumer, /*ShouldOwnClient=*/false);
     if (!ScanInstance.hasDiagnostics())
       return false;
 

 private:
   StringRef WorkingDirectory;
   DependencyConsumer &Consumer;
   llvm::IntrusiveRefCntPtr<DependencyScanningWorkerFilesystem> DepFS;
   ScanningOutputFormat Format;
   bool OptimizeArgs;
   bool EagerLoadModules;
   bool DisableFree;
-  llvm::Optional<StringRef> ModuleName;
+  Optional<StringRef> ModuleName;
+  Optional<CompilerInstance> ScanInstanceStorage;
   std::shared_ptr<ModuleDepCollector> MDC;
   std::vector<std::string> LastCC1Arguments;
   bool Scanned = false;
 };
 
 } // end anonymous namespace
 
 DependencyScanningWorker::DependencyScanningWorker(