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

[Dave Bartolomeo via cfe-commits]

dbartol wrote:

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


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