[NFC][Clang] Fix static analyzer tool remark about missing user-defined assignment operator

Reported by Coverity:

Copy without assign
This class has a user-defined copy constructor but no user-defined assignment operator. If the copy constructor is necessary to manage owned resources then a corresponding assignment operator is usually required. If an object of this type is assigned memory leaks and/or use-after-free errors may occur. Note that a compiler-generated assignment operator will perform only a bit-wise copy for any fields that do not have their own assignment operators defined.

Class has user-written copy constructor but no user-written assignment operator

copy_without_assign: Class <unnamed>::DeclUseTracker has a user-written copy constructor <unnamed>::DeclUseTracker::DeclUseTracker(<unnamed>::DeclUseTracker const &) =delete but no corresponding user-written assignment operator.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D148189