Permit return type of @MonotonicNonNull

It should be permitted to write @MonotonicNonNull on a return type.

A method call to such a method behaves analogously to a field of type @MonotonicNonNull. After the client tests the expression against null, the checker can assume that subsequent uses of the expression are non-null (until the receiver might be re-assigned, for a field or zero-arity method; rules for non-nullary methods would be different and might require purity).

To type-check implementations of methods with @MonotonicNonNull return type, the rules on assignment of @MonotonicNonNull to @MonotonicNonNull need to be relaxed.
Currently such an assignment is forbidden, because the two might be in different states.
However, it is acceptable when a method has a single return statement. Consider the following code, which currently does not type-check but should:

import org.checkerframework.checker.nullness.qual.MonotonicNonNull;

public class MonotonicNonNullReturn {

  @MonotonicNonNull Object field;

  @MonotonicNonNull Object getter() {
    return field;
  }

}

A less restrictive rule would be that if the lhs has only one assignment (statically), then the assignment is permitted (even though lhs and rhs are both @MonotonicNonNull). When a routine has only one return statement, this is satisfied.

Start by locating the nullness checker rules for @MonotonicNonNull return types and assignments, then use the MonotonicNonNullReturn example in the issue as the initial reproduction. Done means the example type-checks and the checker permits the stated single-return assignment behavior while preserving the nullness rules for other cases.