Preprocess Apple llvmCore headers to reflect NDEBUG setting. <rdar://12568983>

If an Apple llvmCore build is done without assertions, and a client uses
the llvmCore headers with assertions enabled, or vice versa, then things will
break because some of the structure sizes in the API are different.  Use the
unifdef tool to make the headers unconditionally match the way the llvmCore
libraries were built.

llvm-svn: 174460