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.
