Split ubsan runtime into three pieces (clang 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.

This change also switches us to using -whole-archive for the ubsan runtime
(which is made possible by the above split), and switches us to only linking
the sanitizer runtime into the main binary and not into DSOs (which is made
possible by using -whole-archive).

The motivation for this is to only link a single copy of sanitizer_common
into any binary. This is becoming important now because we want to share
more state between multiple sanitizers in the same process (for instance,
we want a single shared output mutex).

The Darwin ubsan runtime is unchanged; because we use a DSO there, we don't
need this complexity.