[ThinLTO] Add an option to disable (thin)lto internalization.


[ThinLTO] Add an option to disable (thin)lto internalization.

LTO and ThinLTO optimizes the IR differently.

One source of differences is the amount of internalizations that
can happen.

Add an option to enable/disable internalization so that other
differences can be studied in isolation. e.g. inlining.

There are other things lto and thinlto do differently, I will add
flags to enable/disable them as needed.

Reviewers: tejohnson, pcc, steven_wu

Subscribers: mehdi_amini, inglorion, steven_wu, dexonsmith, dang, llvm-commits

Differential Revision: https://reviews.llvm.org/D53294