[llvm] [Support][YamlTraits] Add quoting for keys in textual YAML representation (PR #88763)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

> here are also cases where quoting
isn't strictly necessary, but prevents issues in some cases, for instance strings
consisting of digits only.

What is the issue with just digit only keys? 

https://github.com/llvm/llvm-project/pull/88763