Add driver options to swift to enable MCCAS.

To enable MCCAS, the following driver options have been added

-cas-backend: Enable MCCAS backend in swift, the option
-cache-compile-job must also be used.

-cas-backend-mode=native: Set the CAS Backend mode to emit an object
file after materializing it from the CAS.

-cas-backend-mode=casid: Emit a file with the CASID for the CAS that was
created.

-cas-backend-mode=verify: Verify that the object file created is
identical to the object file materialized from the CAS.

-cas-emit-casid-file: Emit a .casid file next to the object file when
CAS Backend is enabled.

lib/FrontendTool/FrontendTool.cpp

@@ -1672,6 +1672,7 @@ static bool generateCode(CompilerInstance &Instance, StringRef OutputFilename,
   std::unique_ptr<llvm::TargetMachine> TargetMachine =
       createTargetMachine(opts, Instance.getASTContext());
 
+  TargetMachine->Options.MCOptions.CAS = Instance.getSharedCASInstance();
   // Free up some compiler resources now that we have an IRModule.
   freeASTContextIfPossible(Instance);