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

[Miguel Saldivar via llvm-commits]

Saldivarcher wrote:

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

Done :heavy_check_mark: 

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