cheri_compartment: Warn if return type is void or return value is unused

[v01dXYZ]

Fixes #34.

[davidchisnall]

Thanks for doing this. We are in the middle of rebasing our changes on a newer LLVM, so I’d like to hold off merging until that’s done.

The warning would be better if it is explicit about why it’s a bad idea, rather than saying that it’s unsupported. I’d recommend something like:

void return on a cross-compartment call makes it impossible for callers to detect failure.

A fixit hint that changed the type to int and changed return statements to return 0 (and adds one at the end of the function) would be ideal.

[v01dXYZ]

Thank you. I'll look at FixItHint.