[region-isolation] Add the ability for the analysis to emit "unknown error" partition ops in case we detect a case we cannot pattern match.

DISCUSSION: The analysis itself is unable to emit errors. So we achieve the same
functionality by in such cases emitting a partition op that signals to our user
that when they process that partition op they should emit an "unknown pattern"
error at the partition op's instructions.

I have wanted this for a long time, but I never got around to it.

lib/SILOptimizer/Utils/PartitionUtils.cpp

@@ -94,6 +94,10 @@ void PartitionOp::print(llvm::raw_ostream &os, bool extraSpace) const {
     os << "%%" << opArgs[0];
     break;
   }
+  case PartitionOpKind::UnknownPatternError:
+    os << "unknown pattern error ";
+    os << "%%" << opArgs[0];
+    break;
   }
   os << ": " << *getSourceInst();
 }