[flang] Add analyzed assignment to parse tree

Add `typedAssignment` to the `AssignmentStmt` node, analagous to
`typedExpr` in `Expr`. This represents the analyzed form of an assignment.
It may be a subroutine call in the case of a user-defined assignment.
Add `GetAssignment` function to get it from the parse tree node if present.

Original-commit: flang-compiler/f18@675b0b9bb7617231e8e4bfa0c7a63423c74d79a4
Reviewed-on: https://github.com/flang-compiler/f18/pull/841
Tree-same-pre-rewrite: false