[CIR][NFC] Fix build warnings after #154142 (#157724)

A recent change introduced a new dtor kind, which caused a build warning
in the CIR code where this dtor kind wasn't handled in a switch. This
fixes the problem.

Author: andykaylor

clang/lib/CIR/CodeGen/CIRGenFunction.cpp

@@ -655,6 +655,8 @@ void CIRGenFunction::emitDestructorBody(FunctionArgList &args) {
   // we'd introduce *two* handler blocks.  In the Microsoft ABI, we
   // always delegate because we might not have a definition in this TU.
   switch (dtorType) {
+  case Dtor_Unified:
+    llvm_unreachable("not expecting a unified dtor");
   case Dtor_Comdat:
     llvm_unreachable("not expecting a COMDAT");
   case Dtor_Deleting: