[Serialization] ArrayTypeTraitExpr: serialize sub-expression to avoid keeping it undefined

[a.sidorin]

Summary

I have encountered a segfault when I tried to get sub-expression of serialized ArrayTypeTraitExpr. The fix is simple but I have no idea about nice test. Maybe you have any suggestions?

[ABataev]

You need to add tests for serialization/deserialization

[aaron.ballman]

Serialization tests usually come from PCH, so you could probably look in test/PCH to get ideas on how to test this.

[a.sidorin]

Merged into D14326 and committed.

lib/Serialization/ASTReaderStmt.cpp

 void ASTStmtReader::VisitArrayTypeTraitExpr(ArrayTypeTraitExpr *E) {
   VisitExpr(E);
   E->ATT = (ArrayTypeTrait)Record[Idx++];
   E->Value = (unsigned int)Record[Idx++];
   SourceRange Range = ReadSourceRange(Record, Idx);
   E->Loc = Range.getBegin();
   E->RParen = Range.getEnd();
   E->QueriedType = GetTypeSourceInfo(Record, Idx);
+  E->Dimension = Reader.ReadSubExpr();
 }
 
 void ASTStmtReader::VisitExpressionTraitExpr(ExpressionTraitExpr *E) {
   VisitExpr(E);
   E->ET = (ExpressionTrait)Record[Idx++];
   E->Value = (bool)Record[Idx++];
   SourceRange Range = ReadSourceRange(Record, Idx);
   E->QueriedExpression = Reader.ReadSubExpr();

lib/Serialization/ASTWriterStmt.cpp

 }
 
 void ASTStmtWriter::VisitArrayTypeTraitExpr(ArrayTypeTraitExpr *E) {
   VisitExpr(E);
   Record.push_back(E->getTrait());
   Record.push_back(E->getValue());
   Record.AddSourceRange(E->getSourceRange());
   Record.AddTypeSourceInfo(E->getQueriedTypeSourceInfo());
+  Record.AddStmt(E->getDimensionExpression());
   Code = serialization::EXPR_ARRAY_TYPE_TRAIT;
 }
 
 void ASTStmtWriter::VisitExpressionTraitExpr(ExpressionTraitExpr *E) {
   VisitExpr(E);
   Record.push_back(E->getTrait());
   Record.push_back(E->getValue());
   Record.AddSourceRange(E->getSourceRange());