Call TargetMachine::addEarlyAsPossiblePasses from BackendUtil.

Summary:
As of D18614, TargetMachine exposes a hook to add a set of passes that should
be run as early as possible.  Invoke this hook from clang when setting up the
pass manager.

Reviewers: chandlerc

Subscribers: rnk, cfe-commits, tra

Differential Revision: http://reviews.llvm.org/D18617

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@267764 91177308-0d34-0410-b5e6-96231b3b80d8