[NFC] Loop modernization in diagtool

Precommit for https://reviews.llvm.org/D37390

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312545 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 33dbcd3dd4cc5659a91c0aa13256574ecc72c960)
5 files changed