[Driver] Don't implicitly copy std::strings in for-range loop.
[Driver] Don't implicitly copy std::strings in for-range loop.

Found by -Wrange-loop-analysis.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@248495 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang