[ASTReader] Add a DeserializationListener callback for IMPORTED_MODULES

Summary:
Add a callback from ASTReader to DeserializationListener when the former
reads an IMPORTED_MODULES block. This supports Swift in using PCH for
bridging headers.

Reviewers: doug.gregor, manmanren, bruno

Reviewed By: manmanren

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28779

llvm-svn: 292436