Move ownership of Action objects into Compilation.

Summary:
This makes constructing Action graphs which are DAGs much simpler.  It
also just simplifies in general the ownership semantics of Actions.

Depends on D15910.

Reviewers: echristo

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D15911

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@257407 91177308-0d34-0410-b5e6-96231b3b80d8