[Attributor] Change function internalization to not replace uses in internalized callers

The current implementation of function internalization creats a copy of each
function and replaces every use. This has the downside that the external
versions of the functions will call into the internalized versions of the
functions. This prevents them from being fully independent of eachother. This
patch replaces the current internalization scheme with a method that creates
all the copies of the functions intended to be internalized first and then
replaces the uses as long as their caller is not already internalized.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106931