This patch contains code to resolve metadirectives, either during 
parsing or Gimplification.

The dynamic candidate selection algorithm from the OpenMP 5.1 spec is 
implemented in omp_get_dynamic_candidates in omp-general.c, which 
returns a vector containing information on the top-scoring candidate 
variants. The vector always consists of entries with dynamic selectors 
first, followed by a single entry with an all-static selector (which can 
be the default clause if all the other clauses are dynamic). If all 
selectors are static (i.e. OpenMP 5.0), then omp_get_dynamic_candidates 
will return a vector of at most length 1.

If any part of the selectors in the candidate list cannot be resolved at 
the current stage of compilation, an empty list is returned. Note that 
it is possible to resolve metadirectives even with some selectors 
unresolvable as long as those selectors are not part of the candidate list.

omp_context_selector_matches should always return 1 for dynamic 
selectors (since we can generate code to evaluate the condition at any 
time). omp_dynamic_cond, when given a selector, should return just the 
part of it that must be evaluated at run-time.

Metadirectives are resolved in both tree and Gimple form by generating a 
sequence of if..then..else statements that evaluate the dynamic selector 
of each candidate returned from omp_get_dynamic_candidates in order, 
jumping to the directive body if true, to the next evaluation if not.

Kwok