[clang] [Modules] Delay deserialization of preferred_name attribute at r… (PR #122726)

[Viktoriia Bakalova via cfe-commits]

VitaNuo wrote:

Thank you for the review @ChuanqiXu9!

> (1) Hardcoding is generally not good. And even if we have to, e.g., (we don't have an automatic mechanism to check if we need to defer an attribute), let's avoid hardcoding in both reader side and writer side. We can make this by adding a new bit in the record of attribute to indicate if we need to delay it or not in the reader side generally.

Added a `DeferDeserialization` bit to `Attr` class that is set to `true` for the `preferred_name` and to `false` for all other attributes.
Depending on this field, the skip count field is set on the record (to the count of record fields to skip on initial read for the `preferred_name`, and to 0 otherwise).

> (2) Then I think we can keep the signature of readAttr as is, where it will always skip the check for the delay bit and always try to read it. Then we can add a readOrDeferFor(Decl *D) method as you did. The readOrDeferFor will try to read the deferring bits directly and if it is true, it will record the length skipCount and D to PendingDeferredAttributes . Otherwise, it will call readAttr. I think it is more clear.

Yes this makes sense. It seems like one field (skip count) should be enough. 

https://github.com/llvm/llvm-project/pull/122726