[flang] [llvm] Adding parsing support for omp loop, target loop directives (PR #93517)

[Kareem Ergawy via llvm-commits]

ergawy wrote:

> Yes, `target loop` is non-conforming. But some vendor implementations support it and it will be coming with OpenMP 6.0. So, we should accept it. If people like, we may want to emit a warning it a non-conforming feature is being used and is being mapped to `target teams loop`.

@anchuraj sorry for the late review, yesterday was a public holiday.

I was more concerned about the list of allowed clauses and allowed once clauses. There are small differences between the 2 lists for target teams loop and target loop (for example `if` is in `allowedClauses` for `target teams loop` while in `allowedOnceClauses` for `target loop`). Maybe we just copy and paste the list from `target teams loop` or share them somehow.

https://github.com/llvm/llvm-project/pull/93517