[flang] Fortran IR: staged pull request for the "upper layers" of the IR. The

Fortran IR is hierarchical: A Program contains Procedures.  Procedures
contain Basic Blocks. Groups of Basic Blocks can be grouped as Regions.
This structure follows those one finds in SIL and LLVM IR, etc.

Original-commit: flang-compiler/f18@e2291016dffe76728879ba97d0bfadf2fcb557ed
Reviewed-on: https://github.com/flang-compiler/f18/pull/293
Tree-same-pre-rewrite: false