[flang] Fortran IR: lowest layer of the IR hierarchy. Specifically, Basic Blocks

contain Statements.  This layer is still evolving as the IR takes better
shape and other code in the compiler is written.

Original-commit: flang-compiler/f18@e1a282296b4dc90d05acb7d67f536f169fa36a39
Reviewed-on: https://github.com/flang-compiler/f18/pull/294
Tree-same-pre-rewrite: false