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

[Tom Eccles via llvm-commits]

https://github.com/tblah commented:

Thank you for contributing this.

Please could you add tests showing the correct lowering for flush with an argument, without an argument, and with a dynamically optional argument. There are examples in flang/test/Lower/Intrinsics.

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