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

[NAKAMURA Takumi via llvm-commits]

https://github.com/chapuni commented:

@jh7370 Thanks for your review.

> The new class should be a sibling class of RawContentSection, not a child class.

Yes, it makes sense. I started modifying RawContentSection at first, since covmap sections were under it. With my progression, it can be decoupled finally.

The name `CustomRawContentSections` is odd. I'll rename it to `CustomSection` or whatever. It isn't able to override non-`PROGBITS` sections at the moment, though.

Besides, I've updated for the new section can have `Type`, `Flags`, etc., aka `common`. I noticed they couldn't be generalized while I was adopting `obj2yaml` to non-amd64 ELF binaries.

So, the new `CustomSection` will handle its custom fields instead of `Content`.

This aims for `ObjectYAML` utilities to handle 3rd-party blob formats as plugin. I don't think our `covmap` is a part of ELF. (In contrast, I think extensions to ELF should be implemented in ObjectYAML.) My implementation was not decoupled but a part of `ObjectYAML` when I wrote the article.

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