`opt-out` attributes for intrinsics

Right now I’m annotating intrinsics with new nosync function attribute (https://reviews.llvm.org/D65377)

After some discussion it was decided to ask how the community feels about opt-out attribute list for intrinsics?

For a start some attributes that can be added on the list could be: nosync, nofree, nounwind, willreturn.

Suggestions are welcome.

Thanks,
Stefan