[mlir][llvm] Use tablegen for enum conversion.

The revision uses tablegen to convert multiple atomic and comparison
related enums automatically rather than using hand coded functions
in the import and export from and to LLVM IR.

The revision also adds additional binary operation cases to the
AtomicBinOp enum that have not been supported till now. It also
introduces the possibility to define unsupported enum cases that exist
only in LLVM IR and that are not imported into MLIR. These unsupported
cases are helpful to handle sentinel values such as BAD_BINOP that
LLVM commonly uses to terminate its enums.

Reviewed By: Dinistro

Differential Revision: https://reviews.llvm.org/D143189