[kernel] Cabs2cil: improves location in error messages about struct fields.

src/kernel_internals/typing/cabs2cil.ml

tests/syntax/oracle/fam.res.oracle

...s/syntax/oracle/flexible_array_member_invalid1.res.oracle

...s/syntax/oracle/flexible_array_member_invalid2.res.oracle

...s/syntax/oracle/flexible_array_member_invalid3.res.oracle

...s/syntax/oracle/flexible_array_member_invalid4.res.oracle

...s/syntax/oracle/flexible_array_member_invalid5.res.oracle

tests/syntax/oracle/incomplete_struct_field.res.oracle

...ntax/oracle/struct_with_function_field_invalid.res.oracle

tests/value/oracle/empty_base.0.res.oracle