[clang] [flang] [flang][OpenMP] Remove experimental warning (PR #144915)

[Tom Eccles via cfe-commits]

tblah wrote:

I've taken this out of draft.

The only remaining item in #110008 is some improved semantic checks for Loop transformation constructs. The parsing of these constructs works already and a TODO is emitted, but semantic checks should be updated to allow loop transformation constructs in place of Fortran do loops e.g.
```
!$omp parallel do
!$omp unroll
do i = 1, N
...
```

@Stylie777 is already working on this, but his work could conflict with #143715 and #144785 he will likely base his patch upon that other work and post after they are merged. 

I would like to go ahead and merge this now because the loop transformation constructs are an OpenMP 5.1 feature and they do not lead to a compiler crash, only a misleading semantic error.

Is everyone okay with this being merged?

https://github.com/llvm/llvm-project/pull/144915