[flang] Update description of semantic analysis

Semantics.md described the plans for semantic analysis as of some
time ago. Update it to document what has been implemented.

Original-commit: flang-compiler/f18@d32e34067127aaba139ffd6beee2ad9b41c8aa6c
Reviewed-on: https://github.com/flang-compiler/f18/pull/462
Tree-same-pre-rewrite: false