Support unsafe-allow-reachable in constructors

[ericglau]

Related to #702

If a constructor calls a parent constructor with a modifier-like syntax like the below, @custom:oz-upgrades-unsafe-allow-reachable has no effect if that is defined on the constructor:

abstract contract Parent {
    /// @custom:oz-upgrades-unsafe-allow constructor
    constructor() {
        msg.sender.delegatecall('');
    }
}

/// @custom:oz-upgrades-unsafe-allow constructor
contract Child is Parent {
    /// @custom:oz-upgrades-unsafe-allow-reachable delegatecall
    constructor() Parent() {}
}

This is because we are not dereferencing this constructor call, but are always reporting errors in inherited contract constructors. Note that the constructor call is actually a ModifierInvocation that references the parent contract itself, not the parent constructor.

The workaround for users is to define @custom:oz-upgrades-unsafe-allow-reachable at the contract level instead.