Guard SARIF consumer creation with SWIFT_BUILD_SARIF
Declare and define `sarif_diagnostics::createConsumer` only when
`SWIFT_BUILD_SARIF` is set, and guard the `SARIF` case in `DiagnosticHelper` at
the call site.
Argument parsing already rejects '-serialize-diagnostics=sarif' in such
builds with error_serialize_diagnostics_sarif_unsupported_build, so the
case cannot be reached. It aborts with ABORT rather than llvm_unreachable
so that breaking that guarantee crashes clearly in every build.
