[CAS] FileManager::getObjectRefForFileContent should error when no CAS

Update the API to return error when the CASBackedFileSystem is not
correctly configured instead of return `std::nullopt`. There isn't any
error handling can be done when getting `nullopt` in all usecases and it
is nicer to surface the error directly.

Author: cachemeifyoucan

clang/include/clang/Basic/FileManager.h

@@ -279,7 +279,7 @@ class FileManager : public RefCountedBase<FileManager> {
   /// for its contents if supported by the file system, and then closes the
   /// file. If both the buffer and its `cas::ObjectRef` are needed use \p
   /// getBufferForFile to avoid the extra file lookup.
-  llvm::ErrorOr<std::optional<cas::ObjectRef>>
+  llvm::ErrorOr<cas::ObjectRef>
   getObjectRefForFileContent(const Twine &Filename);
 
 private:

clang/lib/Basic/FileManager.cpp

@@ -585,17 +585,17 @@ FileManager::getBufferForFileImpl(StringRef Filename, int64_t FileSize,
   return getBufferImpl(FilePath);
 }
 
-llvm::ErrorOr<std::optional<cas::ObjectRef>>
+llvm::ErrorOr<cas::ObjectRef>
 FileManager::getObjectRefForFileContent(const Twine &Filename) {
   auto getObjectRefImpl =
-      [&](const Twine &Name) -> llvm::ErrorOr<std::optional<cas::ObjectRef>> {
+      [&](const Twine &Name) -> llvm::ErrorOr<cas::ObjectRef> {
     auto F = FS->openFileForRead(Name);
     if (!F)
       return F.getError();
 
     auto *CASFile = dyn_cast<llvm::cas::CASBackedFile>(F->get());
     if (!CASFile)
-      return std::nullopt;
+      return std::make_error_code(std::errc::not_supported);
 
     return CASFile->getObjectRefForContent();
   };

clang/lib/Tooling/DependencyScanning/IncludeTreeActionController.cpp

@@ -700,7 +700,7 @@ IncludeTreeBuilder::finishIncludeTree(CompilerInstance &ScanInstance,
     if (PPOpts.ImplicitPCHInclude.empty())
       return Error::success(); // no need for additional work.
 
-    llvm::ErrorOr<std::optional<cas::ObjectRef>> CASContents =
+    llvm::ErrorOr<cas::ObjectRef> CASContents =
         FM.getObjectRefForFileContent(PPOpts.ImplicitPCHInclude);
     if (!CASContents)
       return llvm::errorCodeToError(CASContents.getError());
@@ -710,7 +710,7 @@ IncludeTreeBuilder::finishIncludeTree(CompilerInstance &ScanInstance,
       PCHFilename = PPOpts.ImplicitPCHInclude;
 
     auto PCHFile =
-        cas::IncludeTree::File::create(DB, PCHFilename, **CASContents);
+        cas::IncludeTree::File::create(DB, PCHFilename, *CASContents);
     if (!PCHFile)
       return PCHFile.takeError();
     PCHRef = PCHFile->getRef();
@@ -897,15 +897,14 @@ Expected<cas::ObjectRef> IncludeTreeBuilder::addToFileList(FileManager &FM,
     Filename = PathStorage;
   }
 
-  llvm::ErrorOr<std::optional<cas::ObjectRef>> CASContents =
+  llvm::ErrorOr<cas::ObjectRef> CASContents =
       FM.getObjectRefForFileContent(Filename);
   if (!CASContents)
     return llvm::errorCodeToError(CASContents.getError());
-  assert(*CASContents);
 
   auto addFile = [&](StringRef Filename) -> Expected<cas::ObjectRef> {
     assert(!Filename.empty());
-    auto FileNode = createIncludeFile(Filename, **CASContents);
+    auto FileNode = createIncludeFile(Filename, *CASContents);
     if (!FileNode)
       return FileNode.takeError();
     IncludedFiles.push_back(