[clang] [CIR] Change the 'inline_kind' enum to have an explicit attribute (PR #215335)

[Erich Keane via cfe-commits]

erichkeane wrote:

> This looks much better, LGTM! Some inline nits.
> 
> Btw, now that `inline_kind` is a proper `EnumAttr`, does this help move towards the `FIXME: remove once FuncOp uses assembly format` in CIRDialect.cpp? A declarative `$inline_kind` should print the bare keyword on its own and let `parseInlineKindAttr`/`printInlineKindAttr` go away. Not asking you to do it here, just want to know if that's where this is heading.

Thanks for the quick review, I'll write the tests/make the changes suggested.

I am NOT intentionally moving us towards that :) This is just in response to our call-site using the attribute spelling rather than custom printing (as it didn't seem worth the effort?).  I think we would only be able to remove those if we opted to stop printing these as a special keyword-esque thing.  

https://github.com/llvm/llvm-project/pull/215335