Make MultiplexASTDeserializationListener part of the API [NFC]

Summary:
This patch moves the MultiplexASTDeserializationListener declaration into a public header.

We're currently using this multiplexer in the cling interpreter to attach another
ASTDeserializationListener during the execution (so, after the MultiplexConsumer is already
attached which prevents us from attaching more). So far we're doing this by patching clang
and making this class public, but it makes things easier if we make this instead just public in
upstream.

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

Reviewed By: bruno

Subscribers: llvm-commits, cfe-commits, v.g.vassilev

Differential Revision: https://reviews.llvm.org/D37475

llvm-svn: 331021