[clang] [clang] Add -mlarge-eh-encoding driver flag (PR #187583)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay commented:

I'd rather not promote this to a stable driver flag. -mllvm options carry no compatibility guarantee, which is what you want while the design isn't settled. Compiler drivers maintain a high stability and it's hard to walk back. We also haven't settled on a large-code-model strategy overall; this flag could end up the wrong shape.
And the realistic users here (Meta and maybe Google; >2GB .text, near-head builds, centrally controlled toolchains) already tolerate -mllvm fine, so the driver-level convenience is thin.

https://github.com/llvm/llvm-project/pull/187583