[CMake] Fix generated Xcode-project ignoring output directory setting for LLDB.framework

Other generators honor the `LIBRARY_OUTPUT_DIRECTORY` target property, but apparently Xcode doesn't. So we call `set_output_directory()` as `llvm_add_library()` would do and this works.
Note that `LIBRARY_OUTPUT_DIRECTORY` is still necessary, because it's used to store and read the target's absolute build directory (while `LLDB_FRAMEWORK_BUILD_DIR` is relative!).

llvm-svn: 363280