[flang] Parallel runtime library design doc (PRIF) (#76088)
The Parallel Runtime Interface for Fortran (PRIF) specifies the interface design for
supporting Fortran's multi-image parallel features in flang.
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Co-authored-by: Katherine Rasmussen <krasmussen@lbl.gov>
Co-authored-by: Damian Rouson <rouson@lbl.gov>
Co-authored-by: Dan Bonachea <dobonachea@lbl.gov>