[clang] [Clang][C2y] Add support for if declarations (N3356 paper) (PR #198244)

[Muhammad Bassiouni via cfe-commits]

================
@@ -1970,6 +2024,13 @@ Sema::ConditionResult Parser::ParseCXXCondition(StmtResult *InitStmt,
   DeclSpec DS(AttrFactory);
   ParseSpecifierQualifierList(DS, AS_none, DeclSpecContext::DSC_condition);
 
+  // The Declarator's DeclarationAttrs only accepts [[]] and keyword attributes;
+  // move any GNU attributes onto the DeclSpec instead.
+  if (!getLangOpts().CPlusPlus)
----------------
bassiounix wrote:

OK so C++ does handle all the cases properly: https://godbolt.org/z/KGfnjdsdf

The reason for not sharing it with C++ is we parse the GNU attribute as part of a `declspec` to handle the special case for `(__attribute__((...)); expr)` in C2y
```cpp
    // Handle '(; expr)', '(__attribute__((...)); expr)' and '([[...]]; expr)'.
    if (Tok.is(tok::semi)) {
      StmtResult Null = Actions.ActOnNullStmt(ConsumeToken());
      if (ParsedAttrs) {
        WarnOnInit();
        *InitStmt = Actions.ActOnAttributedStmt(attrs, Null.get());
      } else
        Diag(Null.get()->getBeginLoc(),
             diag::err_c2y_first_condition_clause_is_not_declaration);
      return ParseCondition(nullptr, Loc, CK, MissingOK);
    }
```

This is not valid C++ case, it's C2y specific case.
So it makes sense to convert it from `declspec` attribute to become an attribute for the declarator itself

Refer back to this in `clang/lib/Parse/ParseDecl.cpp` `Parser::ParseDeclGroup`
```cpp
    // Accept attributes in an init-declarator.  In the first declarator in a
    // declaration, these would be part of the declspec.  In subsequent
    // declarators, they become part of the declarator itself, so that they
    // don't apply to declarators after *this* one.  Examples:
    //    short __attribute__((common)) var;    -> declspec
    //    short var __attribute__((common));    -> declarator
    //    short x, __attribute__((common)) var;    -> declarator
    MaybeParseGNUAttributes(D);
```

So I'm effectively taking it from this form `short __attribute__((common)) var;` as `declspec` to this form `short var __attribute__((common));` to be part of the declarator itself when it's a `ConditionDecl`.

I don't believe that there's any bugs here, let alone a deeper one. It's working as intended and handles the special case for C2y case.

https://github.com/llvm/llvm-project/pull/198244