[flang] Merge pull request flang-compiler/f18#293 from flang-compiler/ir1

Fortran IR: part 1. staged pull request for the "upper layers" of the IR.  The

Original-commit: flang-compiler/f18@f978be5b8ed77bd3ce62ca1b5c99d3bd42863b82
Reviewed-on: https://github.com/flang-compiler/f18/pull/293