[clang] [C++20][Modules] Fix merging of anonymous members of class templates. (PR #155948)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> > I am confused that when we try to merge the field from "b.h", the primary DC should be the definition instead of the declaration: https://github.com/llvm/llvm-project/blob/899fb7f663a973d6cef57533c85884631f08f26b/clang/lib/Serialization/ASTReaderDecl.cpp#L3385C1-L3389C42
> > I think the definition in your case should be fully instantiated in main.cpp and its value of `isFromASTFile` should be false. What's going on here?
> 
> Yeah, so as far I know:
> 
> 1. The `using SI = S<int>;` creates an instance of `ClassTemplateSpecializationDecl` that's not instantiated, and gets serialized to `a.pcm`.
> 2. When `a.pcm` is read into `main.cpp`, the `ClassTemplateSpecializationDecl` instance is deserialized, and marked `isFromASTFile`.
> 3. When `main.cpp` goes to perform the full instantiation of `S<int>`, it [performs the instantiation "in-place" on the deserialized instance](https://github.com/llvm/llvm-project/blob/ab0bb6db39cefa5c27aab0ac6af7c33e652ba28b/clang/lib/Sema/SemaTemplateInstantiate.cpp#L4241-L4243). So the "definition" of `ClassTemplateSpecializationDecl` instance is still the same one that has `isFromASTFile = true`.

Then in this case, although the `ClassTemplateSpecializationDecl` is marked as `isFromASTFile`, but do its children come from ASTFile? Maybe we can check the `CXXRecordDecl ` of the `ClassTemplateSpecializationDecl` or any other child.

https://github.com/llvm/llvm-project/pull/155948