Fix #4652: allow augmenting setters to choose parameter name

[Darkildo]

This PR updates the augmentations specification so that the implicit setter
induced by a variable declaration no longer has a fixed parameter name _.

Previously, the spec said that for augmentation purposes a non-final variable
(or a late variable without an initializer) implicitly defines a setter with a
parameter named _. After the recent changes around _, this made it
impossible for an augmenting setter declaration to choose its own parameter
name.

This PR changes that wording so that the implicit setter is specified only in
terms of its shape and type:

• it has a single required positional parameter
• the parameter type is the type of the variable
• the variable declaration does not determine the parameter name

This restores the intended ability for an augmenting setter declaration to use a
source-level parameter name such as:

augment set i(myName) {
  _i = myName;
}

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[eernstg]

LGTM, but I do think it would be slightly better to not do this. ;-)

[eernstg]

I think you may have been OOO at the time, but the ability to use a parameter declaration whose name is specified as _ to mean "I don't care" was reintroduced with #4669 and #4670. It should not be necessary to introduce the notion of "does not determine the name" for a parameter declaration.

I don't think it actually creates any problems with the specification to rely on "does not determine the name", but it seems simpler to rely on the ability of _ to do the same thing, now that this feature has been restored.