[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: https://llvm.org/svn/llvm-project/cfe/trunk@267756 91177308-0d34-0410-b5e6-96231b3b80d8