[Flang][OpenMP] Fix for unstructured regions in OpenMP constructs - 2

The following changes are made for OpenMP operations with unstructured region,
1. For combined constructs the outer operation is considered a structured
region and the inner one as the unstructured.
2. Added a condition to ensure that we create new blocks only once for nested
unstructured OpenMP constructs.

Tests are added for checking the structure of the CFG.

Note: This is part of upstreaming from the fir-dev branch of
https://github.com/flang-compiler/f18-llvm-project. Code originally reviewed
at https://github.com/flang-compiler/f18-llvm-project/pull/1394.

Reviewed By: vdonaldson, shraiysh, peixin

Differential Revision: https://reviews.llvm.org/D126375