Split ubsan runtime into three pieces (compiler-rt part):
 * libclang_rt-san-* is sanitizer_common, and is linked in only if no other
   sanitizer runtime is present.
 * libclang_rt-ubsan-* is the piece of the runtime which doesn't depend on
   a C++ ABI library, and is always linked in.
 * libclang_rt-ubsan_cxx-* is the piece of the runtime which depends on a
   C++ ABI library, and is only linked in when linking a C++ binary.

The Darwin ubsan runtime is unchanged.

For more details, see Clang change r177605.