PSyclone does not support implicit symbols

[victoria-atkinson]

When Fortran does not have implicit none and it finds an undeclared symbol then it produces the following error:

   raise VisitorError(
psyclone.psyir.backend.visitor.VisitorError: Visitor Error: The following symbols are not explicitly declared or imported from a module and there are no wildcard imports which could be bringing them into scope: 'symbol_name' 

There is no plan to fix this but the issue has been created for reference.

[arporter]

For discussion: should we update the frontend to check for the presence of implicit none? This is slightly tricky as we don't always deal with a whole piece of code.

[sergisiso]

There are some cases where the implicit none is omitted, but the file still have all symbols declared. We would lose the ability to process those.