Driver: Pass some std::strings by reference instead of value
Driver: Pass some std::strings by reference instead of value

No functional change intended. This fixes PR21463.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@221249 91177308-0d34-0410-b5e6-96231b3b80d8

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