This patch adds the Fortran support to the just-committed C/C++ support for the 'enter' clause.

The 'TO'/'ENTER' usage is first stored in a linked list – and
then as attribute to the symbol. I am not sure how to handle it best.
I did went for adding an ENTER_LIST but kept only using the single attribute.

Result: In one diagnostic, I use 'TO or ENTER clause'  in the other,
I can properly use 'TO' or 'ENTER' clause.

This could be kept as is, but to save memory it would be possible
to drop the ENTER_LIST – or, to improve diagnostic, we could try harder
(e.g. by re-walking the list or by another gfc_attribute). Preferences?
If not, I would go for the attached middle way.

Tobias
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