Mirror outputs into CAS using CASOutputBackend

When `-enable-cas` option is used, mirror the output into CAS so it can
be fetched from CAS with the cache key in the future.

lib/Frontend/Frontend.cpp

@@ -26,6 +26,7 @@
 #include "swift/Basic/FileTypes.h"
 #include "swift/Basic/SourceManager.h"
 #include "swift/Basic/Statistic.h"
+#include "swift/Frontend/CachingUtils.h"
 #include "swift/Frontend/CompileJobCacheKey.h"
 #include "swift/Frontend/ModuleInterfaceLoader.h"
 #include "swift/Parse/Lexer.h"
@@ -448,6 +449,15 @@ void CompilerInstance::setupOutputBackend() {
   OutputBackend =
       llvm::makeIntrusiveRefCnt<llvm::vfs::OnDiskOutputBackend>();
 
+  // Mirror the output into CAS.
+  if (supportCaching()) {
+    auto CASOutputBackend = createSwiftCachingOutputBackend(
+        *CAS, *ResultCache, *CompileJobBaseKey,
+        Invocation.getFrontendOptions().InputsAndOutputs);
+    OutputBackend =
+        llvm::vfs::makeMirroringOutputBackend(OutputBackend, CASOutputBackend);
+  }
+
   // Setup verification backend.
   // Create a mirroring outputbackend to produce hash for output files.
   // We cannot skip disk here since swift compiler is expecting to read back
@@ -1043,6 +1053,14 @@ bool CompilerInstance::canImportCxxShim() const {
          !Invocation.getFrontendOptions().InputsAndOutputs.hasModuleInterfaceOutputPath();
 }
 
+bool CompilerInstance::supportCaching() const {
+  if (!Invocation.getFrontendOptions().EnableCAS)
+    return false;
+
+  return FrontendOptions::supportCompilationCaching(
+      Invocation.getFrontendOptions().RequestedAction);
+}
+
 ImplicitImportInfo CompilerInstance::getImplicitImportInfo() const {
   auto &frontendOpts = Invocation.getFrontendOptions();