Reapply "[Flang][OpenMP][Lower] NFC: Move clause processing helpers into the...

Reapply "[Flang][OpenMP][Lower] NFC: Move clause processing helpers into the ClauseProcessor (#85258)" (#85807)

This patch contains slight modifications to the reverted PR #85258 to
avoid issues with constructs containing multiple reduction clauses,
uncovered by a test on the gfortran testsuite.

This reverts commit 9f80444c.