[llvm] [FunctionAttrs] Treat byval calls as only reading ptrs (PR #122618)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

> I think at least for doesNotCapture we'd be better off changing the API to automatically check byval. 

Done, I now see `nocapture` added in the testcase as well. 

@nikic do you think it makes sense to update `CallBase::onlyReadsMemory` with a similar check, or should I keep things as is?

https://github.com/llvm/llvm-project/pull/122618