[clang] Add -fdiagnostics-format=sarif option for future SARIF output

Adds `sarif` option to the existing `-fdiagnostics-format` flag
for intended future work with SARIF diagnostics. Currently issues a warning
against the use of diagnostics in SARIF mode, then defaults to clang style for
diagnostics.

Reviewed By: cjdb, denik, aaron.ballman

Differential Revision: https://reviews.llvm.org/D129886