TableGen: Introduce `llvm::TableGen::Emitter` to register backends

TableGen: Introduce `llvm::TableGen::Emitter` to register backends

`Opt(flag, func, desc)` registers an option into `Action`.

`OptClass<EmitterC>` is also available if `EmitterC(RK).run(OS)` is capable.

`Action` is defined as `ManagedStatic<cl::opt>` to guarantee to be created
when each registration of emitter is invoked.

`llvm::TableGenMain(argv0, MainFn)` invokes `Action` instead of `MainFn`

Differential Revision: https://reviews.llvm.org/D144351