[llvm] [ELFYAML] Introduce `CustomRawContent` (PR #115707)

[James Henderson via llvm-commits]

https://github.com/jh7370 requested changes to this pull request.

I don't know what you're trying to achieve with this PR, but it certainly isn't how I'd expect a new section type to be supported. The new class should be a sibling class of `RawContentSection`, not a child class. As stated in the RFC, `RawContentSection` is intended for use when you are specifying the contents as a binary blob. A new section type that is designed to provide a way to easily describe the sections you discussed in the RFC would not need that functionality.

https://github.com/llvm/llvm-project/pull/115707