SI-8197 clarify overloading resolution with default args

This commit was co-authored with Lukas. His analysis is below.

If there are multiple applicable alternatives, drop those
that use default arguments. This is done indirectly by
checking applicability based on arity
(disallowing tupling to ensure exact arity matching!).
If defaults are required in the application, the arities
won't match up exactly.

TODO this `namesMatch` business is not spec'ed, and is
the wrong fix for SI-4592. We should instead clarify what
the spec means by "typing each argument with an undefined expected type".

What does typing a named argument entail when we don't know what
the valid parameter names are? (Since we're doing overload resolution,
there are multiple alternatives that can define different names.)

Luckily, the next step checks applicability to the individual alternatives,
so it knows whether an assignment is:
 - a valid named argument
 - a well-typed assignment
 - an error (e.g., rhs does not refer to a variable)

I propose the following solution (as a TODO):
check whether a named argument (when typing it in `doTypedApply`)
could be interpreted as an assign; `infer.checkNames` must not use
`UnitType` for its type unless it's a legal assignment

Lukas's analysis:

990fa04 misunderstood the spec of overloading resolution with
defaults. it should not discard applicable alternatives that define
defaults, but only those that use defaults in the given invocation.

bugs were shadowed because the refactoring used `alt.hasDefault` to
check whether the alternative has some parameters with defaults, but
this never worked.

d5bb19f fixed that part by checking the flags of parameters, which
fixed some but but un-shadowed others:

```
object t { def f(x: String = "") = 1; def f(x: Object) = 2 }
scala> t.f("") // should return 1, but returns 2 after d5bb19f
```

The commit message of d5bb19f also mentions that its test "should
fail to compile according to SI-4728", which is another
misunderstanding.

```
class A; class B extends A
object t { def f(x: A = null) = 1; def f(x: B*) = 2 }
t.f()
```

This should return `2`: both alternatives are applicable, so the one
that uses a default is eliminated, and we're left with the vararg one.
SI-4728 is different in that is uses explicit parameters,
```t.f(new B)``` is ambiguous.