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

Summary:
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

llvm-svn: 346140