Some attributes missing on `llvmlite.ir.Function`

[varun-r-mallya]

Feature request

Ability to add attributes such as nofree, nosync, memory(none) to an ir.Function is required as LLVM currently provides this as well.
https://github.com/numba/llvmlite/blob/03e2c9085dadc15eedcac0d8c34244f11c29bb8a/llvmlite/ir/values.py#L884

[gmarkall]

In the triage meeting today, we did discuss getting rid of the frozen set of attributes and recommending that the verifier be run on the IR. However, as far as I can tell, the verifier will ignore unknown attributes, and thereby allow the kind of error that we've been seeking to catch with the use of the frozen set.