[ForceFunctionAttrs] Make options additionally work over entire module

This patch makes the -force-attribute and -force-remove-attribute flags
in the forceattrs pass additionally work over all the functions in a
module rather than on specific functions. I often
find myself dealing with bitcode from projects with arbitrary build
systems that have applied the optnone attribute for various reasons and
having the ability to do this in opt would be quite convenient. It's
possible to remove the attribute by disassembling to textual IR, running
sed, and then running opt over the result, but this option makes things
just a bit easier.

Differential Revision: https://reviews.llvm.org/D158815

Reviewed-By: nikic