[ASTImporter] Add an ImportImpl method to allow customizing Import behavior.

Summary:
We are currently implementing support in LLDB that reconstructs the STL templates from
the target program in the expression evaluator. This reconstruction happens during the
import process from our debug info AST into the expression evaluation AST, which means
we need a way to intercept the ASTImporter import process.

This patch adds an protected ImportImpl method that we can overwrite in LLDB to implement
our special importing logic (which is essentially just looking into a C++ module that is attached to
the target context). Because ImportImpl has to call MapImported/AddToLookup for the decls it
creates, this patch also exposes those via a new unified method and checks that we call it when
importing decls.

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

Reviewed By: martong, a_sidorin

Subscribers: rnkovacs, cfe-commits, lldb-commits, aprantl

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59485

llvm-svn: 359502