[Serialization] Fix warnings

This patch fixes:

  clang/lib/Serialization/ASTWriterStmt.cpp:576:3: error: default
  label in switch which covers all enumeration values
  [-Werror,-Wcovered-switch-default]

  clang/lib/Serialization/ASTReaderStmt.cpp:561:3: error: default
  label in switch which covers all enumeration values
  [-Werror,-Wcovered-switch-default]

Author: kazutakahirata

clang/lib/Serialization/ASTReaderStmt.cpp

@@ -558,8 +558,6 @@ void ASTStmtReader::VisitConstantExpr(ConstantExpr *E) {
       Record.getContext().addDestruction(&E->APValueResult());
     }
     break;
-  default:
-    llvm_unreachable("unexpected ResultKind!");
   }
 
   E->setSubExpr(Record.readSubExpr());

clang/lib/Serialization/ASTWriterStmt.cpp

@@ -573,8 +573,6 @@ void ASTStmtWriter::VisitConstantExpr(ConstantExpr *E) {
   case ConstantResultStorageKind::APValue:
     Record.AddAPValue(E->APValueResult());
     break;
-  default:
-    llvm_unreachable("unexpected ResultKind!");
   }
 
   Record.AddStmt(E->getSubExpr());