[clang] [AST] Support structural equivalence checking of attributes on Decls (PR #168769)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> > I concluded that the local queue introduced in the previous commits isn’t necessary in C mode. Since attribute equivalence is currently checked only for C23, I removed the code entirely.
> > If we later enable attribute equivalence checking in C++ mode, we may need to revisit this and adopt a similar approach. For example, without a separate queue, clang will diagnose the attribute but fail to report the mismatch in the type of member `a` when attempting to merge the following struct:
> > ```
> > struct GuardedBy {
> >   int b __attribute__((guarded_by(lock)));
> >   // The pair of S is added to NonEquivalentDecls, but the pair of Lock isn't. 
> >   struct __attribute__ ((lockable)) Lock {
> >     struct S {
> > #ifdef D1
> >       unsigned a;
> > #else
> >       int a;
> > #endif
> >     } s;
> >   } lock;
> > };
> > ```
> > 
> > 
> >     
> >       
> >     
> > 
> >       
> >     
> > 
> >     
> >   
> > $ clang -cc1 -std=c++20 -emit-pch -o f1.ast -x c++ test.h $ clang -cc1 -std=c++20 -emit-pch -o f2.ast -DD1 -x c++ test.h $ clang -cc1 -std=c++20 -ast-merge f1.ast -ast-merge f2.ast -x c++ test.cpp
> 
> CC @ChuanqiXu9 @Bigcheese as C++ modules maintainers. I am under the impression we need this for both C++ modules support and C23's redefinition checking. If we don't need the more complicated approach right now, I think the current approach is reasonable for C (but CC @erichkeane for awareness of the tablegen changes), I just want to make sure we're not making further work harder for ourselves.

In modules (Serialization), we already have ODR checks to check the redefinitions are the same. So this is not needed for modules.

https://github.com/llvm/llvm-project/pull/168769