The OmpAtomicClause is a variant of a few specific clauses that are used
on the ATOMIC construct. The HINT clause, however, was represented as a
generic OmpClause, which somewhat complicated the analysis of an
OmpAtomicClause.
Introduce OmpHintClause to represent the contents of the HINT clause,
and use it on OmpAtomicClause similarly to how OmpFailClause is used.
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