ubsan: Unbreak ubsan_cxx runtime library on Windows.

This was originally broken by r258744 which introduced a weak reference
from ubsan to ubsan_cxx. This reference does not work directly on
Windows because COFF has no direct concept of weak symbols. The fix is
to use /alternatename to create a weak external reference to ubsan_cxx.

Also fix the definition (and the name, so that we drop cached values)
of the cmake flag that controls whether to build ubsan_cxx. Now the
user-controllable flag is always on, and we turn it off internally
depending on whether we support building it.

Differential Revision: https://reviews.llvm.org/D37882

llvm-svn: 313391