[OpenACC][CIR] Handle 'declare' construct local lowering (&link clause) (#168793)
'declare' is a declaration directive, so it can appear at 3 places:
Global/NS scope, class scope, or local scope. This patch implements ONLY
the 'local' scope lowering for 'declare'.
A 'declare' is lowered as a 'declare_enter' and 'declare_exit'
operation, plus data operands like all others. Sema restricts the form
of some of these, but they are otherwise identical.
'declare' DOES require at least 1 clause for the examples to make sense,
so this ALSO implements 'link', which is the 'simpliest' one. It is ONLY
attached to the 'declare_enter', and doesn't require any additional work
besides a very small addition to how we handle clauses.