[flang][OpenMP] Non-conforming direct call to a declare-variant function variant accepted silently.

Flang does not diagnose the OpenMP DECLARE VARIANT restriction that a function variant may not be called directly in an OpenMP context different from its construct selector set.

OpenMP 6.0 section 9.6 (Declare Variant Directives), Restrictions: "Calling procedures that a declare variant directive determined to be a function variant directly in an OpenMP context that is different from the one that the construct selector set of the context selector specifies is non-conforming."

Please see the following program.

```fortran
module m
implicit none
contains
subroutine base()
!$omp declare variant(variant) match(construct={parallel})
end subroutine

subroutine variant()
end subroutine

subroutine caller()
! Non-conforming: 'variant' is specialized for construct={parallel}, but it
! is called directly outside any parallel construct.
call variant()

! For contrast, this direct call is conforming (inside a parallel construct).
!$omp parallel
call variant()
!$omp end parallel
end subroutine
end module
```

This compiles cleanly with flang -fc1 -fopenmp -fsyntax-only. The expectation is that call variant() outside of parallel region should generate an error.