[Flang][OpenMP] Restrict check to worksharing construct reductions

The outer context private check for reduction variables was firing
for all constructs. This check is not applicable to non-worksharing
constructs.

OpenMP 5.2: Section 5.5.8
A list item that appears in a reduction clause on a worksharing construct
must be shared in the parallel region to which a correspodning worksharing
region binds.

Reviewed By: peixin

Differential Revision: https://reviews.llvm.org/D144824