[Flang] Flang allows array to be specified in both the allocate-list and the SOURCE= specifier

This was discovered by @claude and might be related to #209709.

According to the Fortran 2023 standard, the *source-expr* and *allocate-object* are prohibited from being the same:

> *source-expr* shall not be allocated within the ALLOCATE statement in which it appears; nor shall it depend on the value, bounds, deferred type parameters, allocation status, or association status of any *allocate-object* in that statement.

* test1.f90

```fortran
program main
implicit none
integer, allocatable :: ptr(:)

! ILLEGAL: ptr appears as both the allocatee and the source= expression.
allocate(ptr, source=ptr) ! must not compile

print *, "pass"
end program main
```

* test2.f90

```fortran
program main
implicit none
integer, allocatable :: ptr1(:), ptr2(:)

! ILLEGAL: ptr2 is both an allocatee and an argument in the source= expression.
allocate(ptr1, ptr2, source=identity(ptr2)) ! must not compile

print *, "pass"
contains
function identity(arr) result(res)
integer, allocatable, intent(in) :: arr(:)
integer, allocatable :: res(:)
res = arr
end function identity
end program main
```

I found that GFortran emits a (runtime) error only for test2.f90 and Intel ifx emits an error only for test1.f90. Flang, however, accepts both.

Start by running Flang against the two provided reproducers, test1.f90 and test2.f90, and confirm that both are accepted. Trace the semantic checking for ALLOCATE statements, especially the allocate-list and SOURCE= expression. Done means Flang rejects both invalid programs according to the Fortran 2023 rule, with suitable diagnostic coverage.

Written by the indexing model from the issue text.