Move the Target way of overriding DAG Scheduler to a target hook

Move the Target way of overriding DAG Scheduler to a target hook

Summary:
The previous way of overriding it was relying on calling "setDefault"
on the global registry, which implies global mutable state.

Reviewers: echristo, atrick

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11538

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243388 91177308-0d34-0410-b5e6-96231b3b80d8