Do systematic search through F# compiler for uses of `ILType`s and how `modreq` will impact them

[TIHan]

We need to do a search through the F# compiler where we use ILTypes explicitly and check how a modreq / ILType.Modified could impact their use.

We fixed a bug: #11552 due to one of these. At the moment, this may be the only bug, but in the future there may be more as modreqs could get added to any ILType. We also should be throwing a compiler error when we encounter a modreq: #5464 - that would make these bugs less surprising and a compiler error would be more explicit rather than a potential runtime error.

[aloraman]

Please note, it is not only modreq, but modopt as well. You can encounter the bug with extra pop instruction with C++/CLI code very easily, no need to wait for the future.
E.g. if someone defines a method in C++/CLI as const void, the resulting IL signature will look something like that:
instance void modopt([mscorlib]System.Runtime.CompilerServices.IsConst) [Assembly]Namespace.Type::Method
And consuming this method in F# code will break runtime due to extra pop instuction. No need to use latest versions of .Net, it's broken for older .Net Framework as well.

[dsyme]

Closing in favour of #5464