[Patch, Fortran] PR54301 - add warning for pointer might outlive its target

[Thomas Koenig]

Hi Tobias,

> Build and regtested on x86-64-gnu-linux.
> OK for the trunk?

I would exclude pointers on the lhs of the pointer assignment, to make
sure that warnings for code such as

program main
   integer :: i
   integer, pointer :: ip
   block
     integer, pointer :: jp
     allocate (jp)
     jp = 3
     ip => jp
   end block
   i = ip
   print *,i
end program main

are not emitted.

	Thomas