swift234

[Parser][Qol] improve diagnostic with missing "self." in init

This improvement was reported in
[SR-852](https://bugs.swift.org/browse/SR-852).

before:

```
$ cat t.swift
class A {
    init(x: Int) {}

    convenience init() { init(x: 1) }
}

$ swiftc -c t.swift
t.swift:6:13: error: initializers may only be declared within a type
    init(x: 1)
        ^
t.swift:6:17: error: expected parameter type following ':'
    init(x: 1)
            ^
t.swift:6:17: error: expected ',' separator
    init(x: 1)
            ^
           ,
t.swift:6:17: error: expected parameter type following ':'
    init(x: 1)
            ^
t.swift:6:17: error: expected ',' separator
    init(x: 1)
            ^
                                                                                                                                                       ,
```

after:

```
t.swift:6:9: error: missing 'self.' at initializer invocation
    init(x: 1)
    ^
    self.
```