A clang tool for changing surrouding namespaces of class/function definitions.

Summary:
A tool for changing surrouding namespaces of class/function definitions while keeping
references to types in the changed namespace correctly qualified by prepending
namespace specifiers before them.

Example: test.cc
   namespace na {
   class X {};
   namespace nb {
   class Y { X x; };
   } // namespace nb
   } // namespace na

To move the definition of class Y from namespace "na::nb" to "x::y", run:
   clang-change-namespace --old_namespace "na::nb" \
     --new_namespace "x::y" --file_pattern "test.cc" test.cc --

Output:
   namespace na {
   class X {};
   } // namespace na
   namespace x {
   namespace y {
   class Y { na::X x; };
   } // namespace y
   } // namespace x

Reviewers: alexfh, omtcyfz, hokein

Subscribers: mgorny, klimek, djasper, beanz, alexshap, Eugene.Zelenko, cfe-commits

Differential Revision: https://reviews.llvm.org/D24183

llvm-svn: 281918