[flang][OpenMP] Add semantic check for declare target (#71861)

This patch adds the following check from OpenMP 5.2.

```
If the directive has a clause, it must contain at least one enter clause
or at least one link clause.
```

Also added a warning for the deprication of `TO` clause on `DECLARE
TARGET` construct.

```
The clause-name to may be used as a synonym for the clause-name enter.
This use has been deprecated.
```

Based on the tests for to clause, the tests for enter clause are added.

This patch does not add tests where both to and enter clause are used together.