[flang] Change ExprRef to return reference

ExprRef never returns a null pointer so change it to return a reference.
Add overloading for parser::Variable as they now also hold a typedExpr.
Change some of the functions that the result of ExprRef is passed to so
that they take references instead of pointers.

Original-commit: flang-compiler/f18@2cdab6b166aa71b273b64b1f6cf520d07637b534
Reviewed-on: https://github.com/flang-compiler/f18/pull/424