Don't pass llvm::DenseSet<> by value.

Summary: Fixes massive performance problems on large translation units.

Reviewers: jordan_rose

Reviewed By: jordan_rose

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3471

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206999 91177308-0d34-0410-b5e6-96231b3b80d8