Hi,

this patch fixes PR77558, an ice-on-invalid-code 6/7 regression.

The fix for PR71602 introduced the invalid-code test-case 
c-c++-common/va-arg-va-list-type.c:
...
__builtin_va_list *pap;

void
fn1 (void)
{
   __builtin_va_arg (pap, double); /* { dg-error "first argument to 
'va_arg' not of type 'va_list'" } */
}
...

The test-case passes for x86_64, but fails for aarch64 and ICEs for arm.

The ICE happens because the patch for PR71602 is incomplete. The patch 
tries to be more strict about returning a canonical va_list only for 
actual va_lists, but doesn't implement this for structure va_list types, 
such as we have for arm, aarch64 and alpha.

This patch adds the missing part, and fixes the ICE.

OK for trunk, 6-branch?

Thanks,
- Tom