[OpenACC] Implement 'if' clause for Compute Constructs (#88411)

Like with the 'default' clause, this is being applied to only Compute
Constructs for now. The 'if' clause takes a condition expression which
is used as a runtime value.

This is not a particularly complex semantic implementation, as there
isn't much to this clause, other than its interactions with 'self',
  which will be managed in the patch to implement that.