[Patch] OpenACC: C/C++ - fix async parsing [PR99137]

[Tobias Burnus]

For:
   #pragma acc parallel async(1,2)

avoid with C an ICE for the original tree:
   #pragma acc parallel async(<<< Unknown tree: c_maybe_const_expr 1, 2 >>>)

It did not ICE with C++, but I think the tree does not make sense, either:
   #pragma acc parallel async(<<< Unknown tree: void_cst >>>, 2)

OK for mainline?
(Not a regression; I don't know whether it makes sense to apply to other branches).

Tobias

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