[clang-move] Enable dump all declarations in old header.

Summary:
* Add -dump_dels option to dump all declarations from old header. It
  will allow clang-move used as a frontend to get declarations from
  header. Further more, this will make debugging stuff easier. Currently only
  class/function types are supported.
* Refactoring code a little bit by creating a ClangMoveContext which
  holds all options for ClangMoveTool, which can simplify the code in
  some degree.

Reviewers: ioeric

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D27059

llvm-svn: 287863