This patch adds diagnostic for additional code alongside a nested teams
in a target region.

The diagnostic is happening soon after parsing such that expressions
in clauses are not yet expanded - those would end up before TEAMS
and can be very complicated (e.g. assume an allocatable-returning function).

(The patch diagnoses it in openmp.cc; after trans-openmp.cc it would
already be to late.)

Comments, remarks, suggestions?

Tobias

PS: Something similar is also needed for C/C++ but there templates
and lambda functions might make it harder to implement. In any case,
it has to be done in the FE. Tracked at PR71065
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