[Patch] Fortran: Fixes for pointer function call as variable (PR96896)

[Tobias Burnus]

During some discussion such an example as attached came up:
   f() = 0.0
where 'f' is a function which returns a pointer to an array.
This gets handled as
   _F.D0 => f()
   _F.D0 = 0.0
However, the first line did fail with a rank error as the rank
was taken from the RHS.

Changing this to the LHS express failed due to 'use_assoc',
which added an 'extern' to the variable and 'proc_pointer'
also caused problems – in principle, either problem could
have also occurred for the RHS.

Side effect: The error message is better for rank mismatch
as for 'f() = a' no pointer assignment is involved (in terms
of the user code) but before we had the error message
'Different ranks in pointer assignment'.

OK?

Tobias

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