[C23][Parser] Diagnostic for attribute declaration where statement is required (#146224)

Fixes: https://github.com/llvm/llvm-project/issues/141659

In C23, something like [[/*possible attributes*/]]; is an attribute
declaration, not a statement. So it is not allowed by the syntax in
places where a statement is required, specifically as the secondary
block of a selection or iteration statement.

Therefore, code like the following should give a diagnostic (at least
with -std=c23 -pedantic), but Clang currently does not produce one:
```cpp
int main(void) {
    if (1)
        [[]];
}
```

---------

Signed-off-by: yronglin <yronglin777@gmail.com>
Signed-off-by: Wang, Yihan <yronglin777@gmail.com>
Co-authored-by: Mariya Podchishchaeva <mariya.podchishchaeva@intel.com>

0 files changed, 0 insertions, 0 deletions