F# ignores custom modifiers (modreq/modopt)

F# is ignoring any custom modifiers from importing IL assemblies:
```fsharp
| ILType.Modified(_,_,ty) ->
// All custom modifiers are ignored
ImportILType env m tinst ty
```
This is violating the Ecma spec. We should probably get a list of C#/VB custom modifiers that have CMOD_REQ, and allow those to go through assuming we handle them properly anyway, and then we throw an error on ones that we don't know about; currently, we do not do this.