[flang][OpenMP] Support privatization for single construct

This supports the lowering of private and firstprivate clauses in single
construct. The alloca ops are emitted in the entry block according to
https://llvm.org/docs/Frontend/PerformanceTips.html#use-of-allocas

, and
the load/store ops are emitted in the single region. The data race
problem is handled in OMPIRBuilder. That is, the barrier is emitted in
OMPIRBuilder.

Co-authored-by: Nimish Mishra <neelam.nimish@gmail.com>

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D128596