[flang] [llvm] [flang] Support FLUSH as an intrinsic subroutine (PR #165942)

[Kelvin Li via llvm-commits]

kkwli wrote:

> > I think we need add this non-standard subroutine in https://github.com/llvm/llvm-project/blob/main/flang/docs/Intrinsics.md.
> 
> Done ✔️

Thanks. LG

https://github.com/llvm/llvm-project/pull/165942