Add an internalization step to the ThinLTOCodeGenerator

Add an internalization step to the ThinLTOCodeGenerator

Keeping as much as possible internal/private is
known to help the optimizer. Let's try to benefit from
this in ThinLTO.
Note: this is early work, but is enough to build clang (and
all the LLVM tools). I still need to write some lit-tests...

Differential Revision: http://reviews.llvm.org/D19103

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 267317