C-API for `llvm-dialects` and generated dialects

[vchuravy]

It would be awesome if llvm-dialects could use the tablegen definitions to generate a C-API automatically.
But how to do so is probably worth some discussion.

As an example this is the current IRBuilder API:

https://github.com/llvm/llvm-project/blob/7fe29e585723c8a90e798b3d852f2c54efb6f5dd/llvm/include/llvm-c/Core.h#L3742-L3753

But there are also delightful enums like: https://github.com/llvm/llvm-project/blob/7fe29e585723c8a90e798b3d852f2c54efb6f5dd/llvm/include/llvm-c/Core.h#L60

[nhaehnle]

It'd probably end up in a state similar to the LLVM C API where sometimes: the C API is strictly a wrapper around the C++ API and some features may not immediately be available in the C API.

I could imagine this working as a second set of llvm-dialects-tblgen actions: one to generate an .inc file for a C header, and one to generate an .inc file for a C++ source that implements the functions exposed by the C header by referring back to the C++ functions.

Layering it in this way ensures that the two APIs agree about how operands are represented etc.

Not sure about the enums...

In terms of project management: We (AMD) have no use for it, but I'd be happy to accept such a feature as long as it has testing that's roughly comparable with what's there for the C++ API.