[change-namespace] move template class forward-declarations and don't move fwd-decls in classes.

Summary:
Forward declarations in moved namespaces should be moved back to the old
namespaces. We should also move template class forward declarations.

Also fix a bug that moves forward declarations of nested classes.

Reviewers: bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D27515

llvm-svn: 288908