[clang-move] Only move used helper declarations.

Summary:
Instead of moving all the helper declarations blindly, this patch
implements an AST-based call graph solution to make clang-move only move used
helper decls to new.cc and remove unused decls in old.cc.

Depends on D27674.

Reviewers: ioeric

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D27673

llvm-svn: 290873