[flang] Fotran IR: part 5. Afforestation is the lowering of concrete parse trees

and semantics information into the Fortran IR.

Original-commit: flang-compiler/f18@d1dde126409d53864118b96363979d6dbcbd1b97
Reviewed-on: https://github.com/flang-compiler/f18/pull/297
Tree-same-pre-rewrite: false