IR: Introduce "exception-model" module flag (#220052)
Add an "exception-model" IR module flag intended to replace
TargetOptions::ExceptionModel, such that the ABI is fully
computable from the IR alone. Add the basic documentation and
verifier/linker tests, but doesn't wire up the consumers.
Co-authored-by: Claude (Claude-Opus-4.8) <noreply@anthropic.com>
