[clang] [clang] Introduce '-fint64-is-long-long' (PR #234780)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt requested changes to this pull request.

I don't like this naming - both the LangOpt and the cli arg seem the wrong way round? surely this should be LongLongIsInt64 and -flong-long-is-int64? the current name implies that -fint64-is-long-long would allow int64 to be defined as int128 on platforms where long long is int128?

https://github.com/llvm/llvm-project/pull/234780