[clang] Enable output of SARIF diagnostics

Enables Clang to emit diagnostics in SARIF format when
`-fdiagnostics-format=sarif`. Adds a new DiagnosticConsumer named
SARIFDiagnosticPrinter and a new DiagnosticRenderer named SARIFDiagnostic
to constuct and emit a SARIF object containing the run's basic diagnostic info.

Reviewed By: cjdb, denik, aaron.ballman

Differential Revision: https://reviews.llvm.org/D131632