[gfortran] Support INTEGER<->LOGICAL conversion as GNU extension

[Tobias Schlüter]

Tobias Schlüter wrote:
> The patch is sound, but there's one special case where it might break stuff.
> Can you try out if it interacts badly with arithmetic IFs (*) and commit a
> suitable testcase for this along with it, if it doesn't?  I'm not sure how
> well-tested arithmetic-if is.

Oh, and I didn't mean to express any opinion on the political debate
surrounding this patch, and therefore I'll ask you not to commit it before
some kind of consensus has been found.

- Tobi