Multiple Constructors require different syntax

_Originally from here: https://github.com/fsharp/fsharp/issues/827_

When adding constructors to to an F# class the required syntax changes, e.g. is different for the last constructor (for single parameter `.ctor`s).

#### Repro steps

While the following code is valid…
```F#
type A (a : int option, b : string option) =
new a = A (a, None)
member __.X = ""
```

… adding another `.ctor` with one parameter in a similar fashion does not validate anymore:

```F#
type A (a : int option, b : string option) =
new a = A (a, None)
new b = A (None, b)
member __.X = ""
```

The error on the `=` on the third line is: `error FS0010: Unexpected symbol '=' in expression`

#### Known workarounds

Parameters of the not-last `.ctor` must be wrapped in braces. For a yet bigger example this gives:

```F#
type A (a : int option, b : string option, c : float option) =
new (a) = A (a, None, None)
new (b) = A (None, b, None)
new c = A (None, None, c)
member __.X = ""
```

(Also omitting the first brace (in `new (a)`) does break the thing.)

#### Related information

Observed on Win 10 in VS and Ionide with latest releases installed.

Severity is most probably low.