[flang][OpenMP] Lowering support for lastprivate on unstructured sections

construct

This patch adds lowering support for lastprivate privatization on
unstructured construct. Generic lastprivate lowering on sections
involves creating a if operation on the lexically last section block
and updating the lastprivate variable inside it. However, this control
flow is not needed if the section construct is lowered as an
unstructured construct. Hence, this patch modifies the lowering control
flow to not emit an unnecessary scf.if if the section block is lowered
as an unstructured construct.

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D143077