[Patch, Fortran] PR30973, disallow to reuse a module name or to use the module name on an ONLY clause of USE

[Tobias Burnus]

I missed some part; gcc/testsuite/gfortran.dg/used_dummy_types_7.f90 had
to be updated as "atom" conflicts now in

SUBROUTINE dummy_atom_list_insert(this, atom)
     USE atom, ONLY: dummy_atom
    TYPE(dummy_atom), INTENT(in)         :: atom

gfortran prints with the attached patch applied:
   Error: Name 'atom' at (1) is an ambiguous reference to 'atom' from
current program unit

This is by the way in line with g95, NAG f95 and ifort.

Updated patch below. Full bootstrap and check-gfortraned on
x86_64-unknown-linux-gnu.

Tobias
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