Adding options for Clang to write diagnostics to file

[cjdb]

@dblaikie

especially given the complexity and the speculative nature of this

While the “better diagnostics” effort as a whole is certainly speculative, I think SARIF is substantially less so.

@dblaikie

(also - we’d probably want to build a decent rendering of diagnostics into clang itself rather than requiring a separate tool if someone’s going to do clang | render_diagnostics, basically - though, yeah, I guess if it’s an ncurses interface or full GUI, that might be beyond what we’d want to include in clang)

I’m not sure what kind of a SARIF consumer Clang could directly support. Getting Clang to produce “better” text-based diagnostics was a much older design (see the ‘Alternative designs’ section), but that was quickly scrapped due to pretty diagnostics not being particularly compatible with command-lines.