Pass the error string directly to llvm_unreachable instead of the residual

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@91644 91177308-0d34-0410-b5e6-96231b3b80d8