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

llvm-svn: 267764