TableGen backends as plugins

[DavidSpickett]

Currently all backends are built into each tblgen binary, llvm-tblgen, clang-tblgen, lldb-tblgen etc. It would be good to have the ability to load a new backend as a plugin.

This would:

• Remove the need for several copies of the compiler (which can be confusing to developers and packagers of llvm).
• Allow faster development of backends in C++.

You can currently write a backend externally by emitting JSON and parsing that with whatever you want, but you won't have the TableGen APIs you'd have in C++.

[DavidSpickett]

https://github.com/llvm/llvm-project/blob/ded255e56ee1f2ef27e85b013f572fca34ca57bc/llvm/utils/TableGen/jupyter/sql_query_backend.ipynb shows the JSON method.