Use @rpath as LC_ID_DYLIB for ASan dylib on OS X

Change the LC_ID_DYLIB of ASan's dynamic libraries on OS X to be set to "@rpath/libclang_rt.asan_osx_dynamic.dylib" and similarly for iossim. Clang driver then sets the "-rpath" to be the real path to where clang currently has the dylib (because clang uses the relative path to its current executable). This means if you move the compiler or install the binary release, -fsanitize=address will link to the proper library.

Reviewed at http://reviews.llvm.org/D6018

llvm-svn: 221278