Refactor the Internalize stage of libLTO in a separate file (NFC)

This is intended to be shared by the ThinLTOCodeGenerator.

Note that there is a change in the way the verifier is run, previously
it was ran as a Pass on the merged module during internalization.
While now the verifier is called explicitely on the merged module
outside of the internalize "pass pipeline".

What remains strange in the API is the fact that `DisableVerify` in
the API does not disable this initial verifier.

Differential Revision: http://reviews.llvm.org/D19000

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 266047