[clang-move] Only move used helper declarations.
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.
Subscribers: mgorny, cfe-commits
Differential Revision: https://reviews.llvm.org/D27673