[Flang][OpenMP] only assertion-enabled flang can reach llvm_unreachable (cancel directive)

[k-arrows]

Reproducer:

program test
!$omp cancel parallel do
end

With assertion-enabled flang, the following output is obtained:

Unexpected directive
UNREACHABLE executed at /path_to_project/llvm-project/flang/lib/Semantics/check-omp-structure.cpp:2521!

The location of this llvm_unreachable is:

llvm-project/flang/lib/Semantics/check-omp-structure.cpp

Line 2521 in 809f857

llvm_unreachable("Unexpected directive");

[llvmbot]

@llvm/issue-subscribers-flang-frontend

Author: None (k-arrows)

Reproducer: ```f90 program test !$omp cancel parallel do end ```

With assertion-enabled flang, the following output is obtained:

Unexpected directive
UNREACHABLE executed at /path_to_project/llvm-project/flang/lib/Semantics/check-omp-structure.cpp:2521!

The location of this llvm_unreachable is:

llvm-project/flang/lib/Semantics/check-omp-structure.cpp

Line 2521 in 809f857

llvm_unreachable("Unexpected directive");