[clang] [clang][diagnostics] Add `-fdiagnostics-add-output` switch for SARIF (PR #185201)

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Sirraide wrote:

> Either of you want to weigh in on this, since you've been interested in previous diagnostics-related PRs?

I haven’t really looked into our SARIF implementation that much... Aaron is more familiar w/ this part of the diagnostics engine than me I believe


https://github.com/llvm/llvm-project/pull/185201