[flang] Fix link error from last integration

We need to be able to analyze the `Variable` in a `Selector` and an
expression. That worked in a previous iteration of `expression.h` by
analyzing each variant of `Variable`.

This fix adds an explicit public function to analyze a Variable and
return a `MaybeExpr`.

Original-commit: flang-compiler/f18@9848a5d48f61f969f11814749c517990f76f6b04
Reviewed-on: https://github.com/flang-compiler/f18/pull/262