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/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -380,6 +380,17 @@ bool ArgsToFrontendOptionsConverter::convert(
     Opts.BlocklistConfigFilePaths.push_back(A);
   }
 
+  if (Arg *A = Args.getLastArg(OPT_cas_backend_mode)) {
+    Opts.CASObjMode = llvm::StringSwitch<llvm::CASBackendMode>(A->getValue())
+                          .Case("native", llvm::CASBackendMode::Native)
+                          .Case("casid", llvm::CASBackendMode::CASID)
+                          .Case("verify", llvm::CASBackendMode::Verify)
+                          .Default(llvm::CASBackendMode::Native);
+  }
+
+  Opts.UseCASBackend = Args.hasArg(OPT_cas_backend);
+  Opts.EmitCASIDFile = Args.hasArg(OPT_cas_emit_casid_file);
+
   return false;
 }