`AttributeUsage` doesn't distinguish `AttributeTargets.Class` and `AttributeTargets.Struct`

[auduchinok]

open System

[<AttributeUsage(AttributeTargets.Class)>]
type CustomClassAttribute() =
    inherit Attribute()

[<AttributeUsage(AttributeTargets.Struct)>]
type CustomStructAttribute() =
    inherit Attribute()

[<AttributeUsage(AttributeTargets.Method)>]
type CustomMethodAttribute() =
    inherit Attribute()

[<CustomClass; CustomStruct; CustomMethod>]
type Class() = class end

[<CustomClass; CustomStruct; CustomMethod>]
type Struct(x: int) = struct end

Struct and class attributes seem to be allowed to be used interchangeably:

[dsyme]

@auduchinok Could you check the title of this bug please? It says .Constructor but doesn't mention that in the description?

Seems there are three problems:

• CustomStruct can be targeted at class incorrectly
• CustomClass can be targeted at struct incorrectly
• CustomMethod can be targeted at a constructor incorrectly

This seem ok:

• CustomConstructor can't be targeted at a method

Example this works but shouldn't:

open System
[<AttributeUsage(AttributeTargets.Method)>]
type CustomMethodAttribute() =
    inherit Attribute()

type Class [<CustomMethod>] () = class end

type Struct [<CustomMethod>] (x: int) = struct end```
and this works (as expected:
```fsharp

[<AttributeUsage(AttributeTargets.Constructor)>]
type CustomConstructorAttribute() =
    inherit Attribute()

type Class [<CustomConstructor>] () = class end

type Struct [<CustomConstructor>] (x: int) = struct end

[auduchinok]

@dsyme Yes, I think it was about structs.