[clang-tidy] Don't warn implicit variables in peformance-unnecessary-copy-initialization.

Summary:

This will prevent the check warning the variables which have been
implicitly added by compiler, like the following case (in for-range loop):

  the variable '__end' is copy-constructed from a const reference...

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D25911

llvm-svn: 286186