Use unique_ptr for cached tokens for default arguments in C++.

Summary:
This changes pointers to cached tokens for default arguments in C++ from raw pointers to unique_ptrs.  There was a fixme in the code where the cached tokens are created  about using a smart pointer.

The change is straightforward, though I did have to track down and fix a memory corruption caused by the change.  memcpy was being used to copy parameter information.  This duplicated the unique_ptr, which led to the cached token buffer being deleted prematurely.

Patch by David Tarditi!

Reviewers: malcolm.parsons

Subscribers: arphaman, malcolm.parsons, cfe-commits

Differential Revision: https://reviews.llvm.org/D26435

llvm-svn: 287241