This is one of two alternative designs that Clang can adopt when writing
SARIF to file. In this design, the Clang driver invokes a separate tool
that "links" the independent diagnostic files into a single one called
<target>_sarif.
Please discuss the design of this patch over at
https://discourse.llvm.org/t/adding-options-for-clang-to-write-diagnostics-to-file/67762/,
alongside <OTHER CL>, so that we can incorporate one of them into D145284.
I ask that commentary regarding the implementation be held off until
either this patch or <OTHER CL> is integrated into D145284.
Depends on D145284.