[CMake] On Darwin bootstrap LTO builds set DYLD_LIBRARY_PATH instead of using llvm-ar
[CMake] On Darwin bootstrap LTO builds set DYLD_LIBRARY_PATH instead of using llvm-ar

llvm-ar isn't really supported for Darwin, instead the host tools will load libLTO, so we can use the just-built libLTO.

This actually makes Darwin bootstrap builds a little faster because you don't need to build llvm-ar before starting the next stage.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@267756 91177308-0d34-0410-b5e6-96231b3b80d8

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