Driver: Update devtoolset usage for RHEL
Driver: Update devtoolset usage for RHEL

- remove path to dts-1.x (corresponds to gcc 4.7)
- add path to dts-6 (corresponds to 6.x)

Patch By: Maria Gottschalk

Differential Revision: https://reviews.llvm.org/D29855

git-svn-id: 42672b9bb47426b7a676b2951937ee8d5bc5f62c@304292 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang