[flang] use setsid to assign the child to prevent zombie as it will be clean up by init process (#77944)

When using `setsid()` in a child process created by `fork()`, a new
session is created, and the child becomes a session leader. If the
parent process terminates before the child, the child becomes an orphan
and is adopted by the `init` process. The `init` process will eventually
clean up the child process once it exits.

However, killing the parent does not automatically kill the child; the
child will continue running until it exits.
Proper cleanup involves waiting for the child process to exit using
`wait()` or `waitpid()` in the parent process to avoid zombie processes,
but this approach is not valid for `EXECUTE_COMMAND_LINE` with async
mode.
Fix: https://github.com/llvm/llvm-project/issues/77803

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