[llvm] [llvm][Support] Add YAMLGenerateSchema for producing YAML schemas (PR #133284)

[Scott Linder via llvm-commits]

slinder1 wrote:

I can see some value in the schema support, but I think the key things I would want a commitment on before being comfortable saying "I would approve a future version of the patchset" are:

* Concrete plan to expose this functionality, including:
  * Documentation, including references to the schema format being generated. If the schema format is not itself well-defined then it feels like adding this support just kicks the can of discoverability down the road, with the user still stuck holding the bag.
  * CLI options in relevant tools to produce schemas.
* Commitment to refactoring of the YAMLIO code to make the new backwards-incompatible mode a clear error when applied in situations where it is broken. The current code just tweaks some interfaces to allow contorting the new mode to fit under the existing `input`/`output` distinction. From my current understanding this is a hack, and I would not want it to land as-is.

I also don't think I can commit to super responsive reviews, but I will try to do better there.

https://github.com/llvm/llvm-project/pull/133284