Pass the error string directly to llvm_unreachable instead of the residual

(0 && "error").  Rough consensus seems to be that g++ *should* be diagnosing
this because the pointer makes it not an ICE in c++03.  Everyone agrees that
the current standard is silly and null-pointer-ness should not be based on
ICE-ness.  Excellent fight scene in Act II, denouement weak, two stars.

llvm-svn: 91644