Simplify OpenMP Lambda use

* Use default ref capture for non-escaping lambdas (this makes
  maintenance easier by allowing new uses, removing uses, having
  conditional uses (such as in assertions) not require updates to an
  explicit capture list)
* Simplify addPrivate API not to take a lambda, since it calls it
  unconditionally/immediately anyway - most callers are simply passing
  in a named value or short expression anyway and the lambda syntax just
  adds noise/overhead

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D121077