Filter object file from CASOutputBackend in MCCAS.

When MCCAS is used, filter the object file from the CASOutputBackend
because we do not want to write the object file in the CAS, we want
to associate the MCCAS top level CAS Object to the main output of the
CASOutputBackend.

Author: rastogishubham

lib/Frontend/Frontend.cpp

@@ -474,10 +474,31 @@ void CompilerInstance::setupOutputBackend() {
 
   // Mirror the output into CAS.
   if (supportCaching()) {
+    auto &InAndOuts = Invocation.getFrontendOptions().InputsAndOutputs;
     CASOutputBackend = createSwiftCachingOutputBackend(
-        *CAS, *ResultCache, *CompileJobBaseKey,
-        Invocation.getFrontendOptions().InputsAndOutputs,
+        *CAS, *ResultCache, *CompileJobBaseKey, InAndOuts,
         Invocation.getFrontendOptions().RequestedAction);
+
+    if (Invocation.getIRGenOptions().UseCASBackend) {
+      auto OutputFiles = InAndOuts.copyOutputFilenames();
+      std::unordered_set<std::string> OutputFileSet(
+          std::make_move_iterator(OutputFiles.begin()),
+          std::make_move_iterator(OutputFiles.end()));
+      // Filter the object file output if MCCAS is enabled, we do not want to
+      // store the object file itself, but store the MCCAS CASID instead.
+      auto FilterBackend = llvm::vfs::makeFilteringOutputBackend(
+          CASOutputBackend,
+          [&, OutputFileSet](StringRef Path,
+                             std::optional<llvm::vfs::OutputConfig> Config) {
+            if (InAndOuts.getPrincipalOutputType() != file_types::ID::TY_Object)
+              return true;
+            return !(OutputFileSet.find(Path.str()) != OutputFileSet.end());
+          });
+      OutputBackend =
+          llvm::vfs::makeMirroringOutputBackend(OutputBackend, FilterBackend);
+      return;
+    }
+
     OutputBackend =
         llvm::vfs::makeMirroringOutputBackend(OutputBackend, CASOutputBackend);
   }