[llvm] [YAMLParser] Improve plain scalar spec compliance (PR #68946)

[Scott Linder via llvm-commits]

https://github.com/slinder1 requested changes to this pull request.

Thank you for the patch!

I am sorry for the delay in review, I was out most of last week.

The change looks good overall, but there are a couple test changes/additions I'd like, and I am a bit confused around some of the more subtle conditions and hoped you could just help me understand.

https://github.com/llvm/llvm-project/pull/68946