Ping **(5.0/7.0) Mark variables in references for variable definition context as used.

Ping?

(The false positives are technically regressions. I didn't file a
separate PR, though).

> what it says in the ChangeLog entry. This fixes a few false positives.
> 
> Regression-tested. OK for trunk?
> 
> Best regards
> 
>      Thomas
> 
> Mark variables in references for variable definition context as used.
> 
> Code like "a(i) = 42" would not mark i as used, leading to false
> positives for warnings with -Wunused-but-set-variable.  This is fixed
> in the attached patch.  It also removes some default arguments to
> make sure that the caller side provides correct information.
> 
> gcc/fortran/ChangeLog:
> 
>      PR fortran/30438
>      * gfortran.h (gfc_value_set_at): Remove default argument.
>      (gfc_expr_set_at): Add prototype.
>      * interface.cc (gfc_compare_actual_formal): Use gfc_expr_set_at.
>      * intrinsic.cc (mark_args_as_used): Likewise.
>      * io.cc (resolve_tag): Likewise.
>      (gfc_resolve_dt): Likewise.
>      (gfc_resolve_inquire): Likewise.
>      * resolve.cc (resolve_transfer): Likewise.
>      (mark_lhs_assignments_set): Likewise.
>      * symbol.cc (gfc_lvalue_allocated_at): Add comment, fix formatting.
>      (gfc_expr_set_at): New fuction.
> 
> gcc/testsuite/ChangeLog:
> 
>      PR fortran/30438
>      * gfortran.dg/warn_unused_but_set_variable_2.f90: New test.