[flang][openacc] Update loop construct lowering

Update the loop construct lowering to support multiple occurences of the same clauses
such as private. Add some utility functions used by other constructs.

Upstreaming part of https://github.com/flang-compiler/f18-llvm-project/pull/438/

Reviewed By: schweitz

Differential Revision: https://reviews.llvm.org/D88253