[clang] Extend diagnose_if to accept more detailed warning information, take 2 (#119712)

This is take two of #70976. This iteration of the patch makes sure that
custom
diagnostics without any warnin

[clang] Extend diagnose_if to accept more detailed warning information, take 2 (#119712)

This is take two of #70976. This iteration of the patch makes sure that
custom
diagnostics without any warning group don't get promoted by `-Werror` or
`-Wfatal-errors`.

This implements parts of the extension proposed in
https://discourse.llvm.org/t/exposing-the-diagnostic-engine-to-c/73092/7.

Specifically, this makes it possible to specify a diagnostic group in an
optional third argument.

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