Set completion priority of destructors and operators to CCP_Unlikely.

Summary:
It will move destructors and operators to the end of completion list.
Destructors and operators are currently very high on the completion
list, as they have the same priority as member functions. However,
they are clearly not something users usually choose in completion
lists.

Reviewers: arphaman, erikjv, bkramer, krasimir

Reviewed By: arphaman

Subscribers: eraman, klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D38081

llvm-svn: 314019