Disable the ubsan-asan config on Darwin.
UBSan doesn't work together with ASan on Darwin yet, see http://llvm.org/bugs/show_bug.cgi?id=21112


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@218746 91177308-0d34-0410-b5e6-96231b3b80d8