DefaultValue attribute should require 'mutable' when used in classes and records

For structs, adding the DefaultValue attribute to a val declaration (a field) results in a check that the field is mutable, since it doesn't make sense to use an immutable field which only ever has the default value.

``` fsharp
[]
type S() =
[] val x : C
```

error FS0880: Uninitialized `val` fields must be mutable and marked with the `[]` attribute. Consider using a `let` binding instead of a `val` field.

For some reason, this condition is only checked for fields declared in structs. We should likewise give a warning for records and classes:

``` fsharp
type C() =
[] val x : C
```

and

``` fsharp
type R = { [] x : C; y : int }
```
## Response

*\* by fslang-admin on 9/8/2015 12:00:00 AM **

Approved for inclusion in a future release of F#.
Please consider contributing to F# by providing an implementation, with adequate testing. The code to be adjusted is around here: https://github.com/fsharp/fsharp/blob/212c3359bf6d83e30c12e53fd2ef283d3257b328/src/fsharp/PostInferenceChecks.fs#L1422. For some reason, the code is only activated for struct/enum type definitions, but it should also apply to record and class type definitions.
Don Syme, F# Language Evolution

**[Original UserVoice Submission](https://fslang.uservoice.com/forums/245727-f-language/suggestions/9679206)**