[Flang][OpenMP] Implement OpenMP 5.2 metadirective

metadirective in flang is implemented in parsing and sema but the lowering is not done yet.

```fortran
subroutine static_vendor()
!$omp metadirective &
!$omp & when(implementation={vendor(llvm)}: barrier) &
!$omp & otherwise(nothing)
end subroutine
```

error: loc("/ptmp/cchen/metadirective/metadirective_static.f90":1:1): /ptmp/cchen/chichun-llvm-project/flang/lib/Lower/OpenMP/OpenMP.cpp:4201: not yet implemented: METADIRECTIVE
LLVM ERROR: aborting