[llvm] Add support for CHERI "purecap" as an environment in target triples. (PR #118927)

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

Also, you can’t just go upstreaming random bits of CHERI support. You need to talk to us first about it as CHERI LLVM maintainers, and you need to talk to the upstream LLVM community first. Triples aren’t exactly useful without something behind them…

https://github.com/llvm/llvm-project/pull/118927