[PATCH] Add llvm::function_ref as a type-erased non-owning reference to a callable

[Rafael Espíndola]

> Revised patch attached. This also converts optimizeGlobalCtorsList to use
> function_ref.

I would probably keep isEmptyFunction as a stand alone function with a
comment instead of converting it to a lambda.

This is quiet handy, it should have an entry in the ProgrammersManual.

LGTM with those changes.

Cheers,
Rafael