[flang] Better language feature enablement and warnings.

Original-commit: flang-compiler/f18@359e4d2c600a31f26774a46f01b2a70fc62bc558
Reviewed-on: https://github.com/flang-compiler/f18/pull/130
Tree-same-pre-rewrite: false
12 files changed
tree: 553d88d998642c15a0075756280e2ac55857c0c9
  1. flang/