clang: Simplify OpenMP triple adjustment (#189265)
Previously this would find a list of offloading triples,
then later fill in the unknown components specifically for
OpenMP after the fact. Start normalizing the triples upfront,
before inserting into the set. Also stop special casing OpenMP
since there's no apparent reason to treat it differently from
other offload languages.
Also operate on the Triple rather than the string, and handle
the unset OS and environment separately.
