Add getDeserializationListener to ASTReader

Summary:
We need to know if ASTReader already has a DeserializationListner or
not, and this also helps to create a multiplexing deserialization
listener if there is one already attached.

Reviewers: v.g.vassilev, rsmith, dblaikie, thakis

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D45921

llvm-svn: 330946