[Flang][OpenMP] Spurious error for atomic update of complex variable with a complex expression

Flang incorrectly rejects valid OpenMP atomic update code when the expression involves multiple additions.

Expected Behavior: The atomic update statement x = x + Y + Z should be accepted. According to the Fortran standard, X + Y + Z and X + (Y + Z) are equivalent forms. Since Flang accepts x = x + (Y + Z) without error, it should also accept x = x + Y + Z.

Actual Behavior: Flang produces the following errors:
```
error: The atomic variable x cannot be a proper subexpression of an argument (here: x+(0._4,1._4)) in the update operation
x = x + (0,1) + f()
error: The atomic variable x should appear as an argument of the top-level + operator
x = x + (0,1) + f()
```
Reproducer:

```fortran
Program test
Complex :: x
x = 0
!$OMP PARALLEL num_threads(2)

!$OMP ATOMIC
x = x + (0,1) + f()
!$OMP ATOMIC
x = x + (0,1) + f()
!$OMP END PARALLEL
if (x/=(4,4)) print *,x,'bug'
print '("ok")'
contains
Integer function f()
Complex :: w=(1,1)
f = w
End function
End
```