Add llvm_unreachable after fully covered switch (NFC)

This fixes a compiler warning for some version of GCC.

mlir/lib/Target/Cpp/TranslateToCpp.cpp

@@ -668,6 +668,7 @@ bool CppEmitter::shouldMapToUnsigned(IntegerType::SignednessSemantics val) {
   case IntegerType::Unsigned:
     return true;
   }
+  llvm_unreachable("Unexpected IntegerType::SignednessSemantics");
 }
 
 bool CppEmitter::hasValueInScope(Value val) { return valueMapper.count(val); }