Don't block non-blittable pointer types in P/Invokes. (#1866)

Don't block non-blittable pointer types in P/Invokes. (#1866)

* Don't block non-blittable pointer types in P/Invokes.

Don't block non-blittable pointer types in P/Invokes. If you're using pointers in interop you're already using unsafe code so we're going to assume you know what you're doing. C# won't let you form a pointer to a reference, so there's no risk of passing an object reference directly to native unless you're writing IL directly.

Fixes dotnet/coreclr#27800

* Delete unused resources.

* Apply fix to managed type system as well.

* Add positive test for nonblittable pointers. Remove negative tests that were in GenericTest.