[flang] Distinguish between unanalyzed and error expressions

When an Expr or Variable is analyzed, always fill in a GenericExprWrapper
for it. That now holds the result of expression analysis which is
std::nullopt when there is an error.

After the ExprChecker pass has finished, the typedExpr data member of
Variables and top-level Exprs should be filled in. Assert that is the
case when we access them.

Original-commit: flang-compiler/f18@192a4e08554b105c48fcd65e6f8ed4860e64f544
Reviewed-on: https://github.com/flang-compiler/f18/pull/424
Tree-same-pre-rewrite: false