sprinkle llvm_unreachable for covered switches (NFC)

Annotate the covered switches with `llvm_unreachable` to avoid the MSVC
warning which does not recognise the covered switches.  This allows us
to avoid a spew of warnings.

lib/Sema/DerivedConformanceAdditiveArithmetic.cpp

@@ -51,6 +51,7 @@ static StringRef getMathOperatorName(MathOperator op) {
   case Subtract:
     return "-";
   }
+  llvm_unreachable("invalid math operator kind");
 }
 
 bool DerivedConformance::canDeriveAdditiveArithmetic(NominalTypeDecl *nominal,