[Clang][OpenMP] Fix wrong form of 'cond-update-stmt' in atomic_ast_print.cpp

In `clang/test/OpenMP/atomic_ast_print.cpp` for `atomic compare capture`,
it was using 'cond-expr-stmt' instead of 'cond-update-stmt'. The spec only supports
'cond-update-stmt'.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D120252