[IR/MachineOutliner] Add a "nooutline" function attr and respect it

Add `nooutline` + update LangRef to say it exists.

This makes it possible to say "don't outline from this function ever."

We want to be able to toggle whether or not a function should be in the search
set regardless of default behaviour.

Add testcases for the IR Outliner + Machine Outliner.

Also remove an unnecessary check for an empty function in the Machine Outliner.

Differential Revision: https://reviews.llvm.org/D140438