[flang] Merge pull request flang-compiler/f18#24 from ThePortlandGroup/sch_sema

Add initial semantic analysis on statements

Original-commit: flang-compiler/f18@68bb9f670ddbaf71ca735d4d7241ef4f67720e22
Reviewed-on: https://github.com/flang-compiler/f18/pull/24
tree: f9e4c732d597be88a7768faf4b365b88dc523006
  1. flang/