[flang] [llvm] [flang][MIF] Adding Stop and ErrorStop operations (PR #166787)

[Peter Klausler via llvm-commits]

klausler wrote:

> Thanks for the update, I will have a look. Adding @klausler for the runtime part.

The code in the runtime for image notification is a place-holder; feel free to replace or improve it.  I thought it would be needed by the runtime library's general error termination mechanism (`Terminator`) so that runtime-detected program errors can fail an image gracefully.  My thinking was that since graceful image termination needs to exist for `Terminator`, it could be used as well for `STOP`/`ERROR STOP`/`FAIL IMAGE` and a main program `END` as well.  Having a PRIF implementation call the runtime instead of the other way around might make it harder to use PRIF with non-flang-new Fortran runtimes, if that's a goal.

https://github.com/llvm/llvm-project/pull/166787