[CAS] Use OnDiskOutputBackend to write outputs

Use a OnDiskOutputBackend to write cached outputs. Currently, the cached
output is written to a FileOutputBuffer, which means if it is writing to
an existing location, the existing file might be removed on creation of
the buffer and create a window of invalid file before the output is
commited.

Using an ondisk output backend (default to atomic write) will make sure
the file didn't get overwritten until the file is completed, thus the
file is always valid.

rdar://137080508
(cherry picked from commit 031a781)

Author: cachemeifyoucan

clang/lib/Frontend/CompileJobCache.cpp

@@ -752,6 +752,7 @@ Expected<std::optional<int>> ObjectStoreCachingOutputs::replayCachedResult(
   DiagnosticsEngine &Diags = Clang.getDiagnostics();
   bool HasMissingOutput = false;
   std::optional<llvm::cas::ObjectProxy> SerialDiags;
+  llvm::vfs::OnDiskOutputBackend Backend;
 
   auto processOutput = [&](clang::cas::CompileJobCacheResult::Output O,
                            std::optional<llvm::cas::ObjectProxy> Obj) -> Error {
@@ -785,47 +786,48 @@ Expected<std::optional<int>> ObjectStoreCachingOutputs::replayCachedResult(
 
     bool IsOutputFile = O.Kind == OutputKind::MainOutput;
 
-    std::optional<StringRef> Contents;
-    SmallString<50> ContentsStorage;
     if (IsOutputFile && ComputedJobNeedsReplay) {
-      llvm::raw_svector_ostream OS(ContentsStorage);
+      auto Output = Backend.createFile(Path);
+      if (!Output)
+        return Output.takeError();
       if (WriteOutputAsCASID)
-        llvm::cas::writeCASIDBuffer(CAS.getID(O.Object), OS);
+        llvm::cas::writeCASIDBuffer(CAS.getID(O.Object), *Output);
       else if (UseCASBackend) {
         // Replay by write out object file.
         // When the environmental variable is set, save the backend CASID for
         // analysis later.
         if (llvm::sys::Process::GetEnv("CLANG_CAS_BACKEND_SAVE_CASID_FILE")) {
           std::string CASIDPath = Path + ".casid";
           std::error_code EC;
-          llvm::raw_fd_ostream IDOS(CASIDPath, EC);
-          if (EC)
-            return llvm::errorCodeToError(EC);
-          writeCASIDBuffer(CAS.getID(O.Object), IDOS);
+          auto IDOut = Backend.createFile(CASIDPath);
+          if (!IDOut)
+            return IDOut.takeError();
+          writeCASIDBuffer(CAS.getID(O.Object), *IDOut);
+          if (auto E = IDOut->keep())
+            return E;
         }
         auto Schema = std::make_unique<llvm::mccasformats::v1::MCSchema>(CAS);
-        if (auto E = Schema->serializeObjectFile(*Obj, OS))
+        if (auto E = Schema->serializeObjectFile(*Obj, *Output))
           return E;
       }
-      Contents = ContentsStorage;
-    } else if (JustComputedResult) {
+      return Output->keep();
+    }
+
+    if (JustComputedResult)
       return Error::success(); // continue
-    } else if (O.Kind == OutputKind::Dependencies) {
-      llvm::raw_svector_ostream OS(ContentsStorage);
+
+    auto Output = Backend.createFile(Path);
+    if (!Output)
+      return Output.takeError();
+    if (O.Kind == OutputKind::Dependencies) {
       if (auto E = CASDependencyCollector::replay(
-              Clang.getDependencyOutputOpts(), CAS, *Obj, OS))
+              Clang.getDependencyOutputOpts(), CAS, *Obj, *Output))
         return E;
-      Contents = ContentsStorage;
     } else {
-      Contents = Obj->getData();
+      *Output << Obj->getData();
     }
 
-    std::unique_ptr<llvm::FileOutputBuffer> Output;
-    if (Error E = llvm::FileOutputBuffer::create(Path, Contents->size())
-                      .moveInto(Output))
-      return E;
-    llvm::copy(*Contents, Output->getBufferStart());
-    return Output->commit();
+    return Output->keep();
   };
 
   if (auto Err = Result.forEachLoadedOutput(processOutput))