This patch adds support for the OpenMP 5.2 syntax for the linear clause,
following the C/C++ patch. The testcases are modified versions from the
C/C++ ones, plus one added one for duplicated modifiers.

At least to me it is not quite clear when
   linear ( var : ref)
refers to a variable 'ref' and when to the linear-modifier 'ref'; the
spec does not seem to be very clear about it. I made an attempt, based
on the C/C++ code – but I am not positive I handle it correctly.

Tested on x86-64-gnu-linux.
OK? Or are there comments?

Tobias
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