Fix warnings in Sema

Author: Sasha Krassovsky

lib/Sema/CSDiagnostics.cpp

@@ -1887,6 +1887,7 @@ getContextualNilDiagnostic(ContextualTypePurpose CTP) {
   case CTP_SubscriptAssignSource:
     return diag::cannot_convert_subscript_assign_nil;
   }
+  llvm_unreachable("Unhandled ContextualTypePurpose in switch");
 }
 
 bool ContextualFailure::diagnoseConversionToNil() const {

lib/Sema/CSSimplify.cpp

@@ -2129,6 +2129,7 @@ static ConstraintFix *fixPropertyWrapperFailure(
       return UseWrappedValue::create(cs, decl, baseTy, toType.getValueOr(type),
                                      locator);
     }
+    llvm_unreachable("Unhandled Fix type in switch");
   };
 
   if (auto storageWrapper = cs.getStorageWrapperInformation(resolvedOverload)) {

lib/Sema/TypeCheckStorage.cpp

@@ -1818,6 +1818,7 @@ SynthesizeAccessorRequest::evaluate(Evaluator &evaluator,
 #include "swift/AST/AccessorKinds.def"
     llvm_unreachable("not an opaque accessor");
   }
+  llvm_unreachable("Unhandled AccessorKind in switch");
 }
 
 llvm::Expected<bool>