[clang] [clang-tools-extra] [Clang] [Diagnostics] Add a new text diagnostics format that supports nested diagnostics (PR #151234)

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Sirraide wrote:

> Thank you for working on this, I think it's a really cool improvement! However, I think it probably also needs an RFC to get more community buy-in, would you mind posting one (the content of the patch summary would suffice)?

Yeah, if a pr’s description gets this long it probably should be an RFC haha.

https://github.com/llvm/llvm-project/pull/151234