Some self-excluding type constraint combinations are allowed, wrong error when they aren't

If you use a constraint combination that isn't allowed, you always get the same error, twice:

> Program.fs(38,76): error FS0043: The constraints 'struct' and 'not struct' are inconsistent
> Program.fs(38,76): error FS0043: The constraints 'struct' and 'not struct' are inconsistent


Repro:

```f#
type Del<'T when 'T: comparison and 'T: delegate> =
member _.Z = 1
```

Some others are allowed, but cannot be instantiated:

```f#
type EnumNull<'T when 'T:enum and 'T:null>()
type StructNull<'T when 'T:'T:struct and 'T:null>()
type DelEnum<'T when 'T: enum and 'T: delegate>
type UnmDel<'T when 'T: unmanaged and 'T: delegate> =
// etc
```

As was mentioned on Slack, some of these are used with SRTP dirty tricks (see [slack discussion](https://fsharp.slack.com/archives/C1R50TKEU/p1595595515248400) with @NinoFloris and an example: https://github.com/fsprojects/FSharpPlus/blob/7e32d52e9881ff40b5440f5ca886b56e6dd4a4c4/src/FSharpPlus/Control/Alternative.fs#L28)

### Proposal: fix

I propose we fix this:

* Fix the error to show the actual conflicting / inconsistent constraints
* Fix the error to show only once
* Make others that shouldn't be allowed (i.e., that cannot be combined in any type) a warning, and add it as default to `warnings-as-errors` (to prevent this from becoming a backward compat issue, similar to how we added FS3239 by default to new projects).