[clang] Sort attributes according to source position before printing (PR #162556)

[Erich Keane via cfe-commits]

erichkeane wrote:

> > > @erichkeane So, looking why the `asm` attribute is written after the visibility attribute, I see on
> > > https://github.com/llvm/llvm-project/blob/92266681bfc89b71d1846b68da296186cd8bfbec/clang/lib/Sema/SemaDecl.cpp#L8128
> > > 
> > > :
> > > ```
> > >   ProcessDeclAttributes(S, NewVD, D);
> > > ```
> > > 
> > > 
> > >     
> > >       
> > >     
> > > 
> > >       
> > >     
> > > 
> > >     
> > >   
> > > which creates the VisibilityAttr. Then later on
> > > https://github.com/llvm/llvm-project/blob/92266681bfc89b71d1846b68da296186cd8bfbec/clang/lib/Sema/SemaDecl.cpp#L8223
> > > 
> > > ```
> > > if (Expr *E = D.getAsmLabel()) {
> > >     <...>
> > >     NewVD->addAttr(AsmLabelAttr::Create(Context, Label, SE->getStrTokenLoc(0)));
> > > }
> > > ```
> > > 
> > > 
> > >     
> > >       
> > >     
> > > 
> > >       
> > >     
> > > 
> > >     
> > >   
> > > Which then creates the `asm` attribute. Hence the ASM attribute is created after visibility attributes at least. The question now is which attributes can be placed before an `asm` attribute which may resulted in this code layout.
> > 
> > 
> > Ah, whew, so it IS just a handful that are inserted incorrectly! That is good to know. The `AsmLabelAttr` being a 'special' thing there is a little frustrating... however, does its order ever matter? I wouldn't expect so? So it at least shouldn't cause any problems, right?
> 
> I just looked at the ISO/IEC 9899:2024 if the language standard does define anything about `asm` and attribute specific layouts and they don't define anything for `asm`, just that it "may be used to insert assembly language directly into the translator output". It doesn't define order of any sort, so this behavior is here probably due to implementation details or to mimic gcc behavior.
> 
> So, switching the order may fix the problem (i.e., print the Asm _before_ the other attributes) but I wonder what other problems this may expose.
> 
> Do you suggest any specific approach here to properly solve this?

I see two options:  Either 1: Always insert Asm 'first'.  Or 2: figure out how to insert Asm in the 'right' place.  

I would lean towards #1, and wonder if we're better off moving the ProcessDeclAttrs after it (or the Asm handling before it?).

@AaronBallman : curious of your thoughts.

https://github.com/llvm/llvm-project/pull/162556