Add User docs for ASTImporter

Summary:
This document includes the description of the ASTImporter from the user/client
perspective.
A subsequent patch will describe the development internals.

Reviewers: a_sidorin, shafik, gamesh411, balazske, a.sidorin

Subscribers: rnkovacs, dkrupp, arphaman, Szelethus, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65573

llvm-svn: 368009