Use const-ref in range-loop for to avoid copying pairs of std::string

No reason to create temporaries.

Differential Revision: https://reviews.llvm.org/D29871

Patch by sergio.martins!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295807 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed