Shuffle some `MethodTable` flags (#85634)

Shuffle some `MethodTable` flags (#85634)

* `IDynamicInterfaceCastableFlag` cannot be set on types that have a component size, so move it from the valuable location in `Flags` to `FlagsEx`.
* Move `HasSealedVTableEntriesFlag` from rare flags to flags. This flag is not so rare. All async state machine types set it, for example.
* Delete `IsAbstractClassFlag`. This was introduce in .NET Native for `GetUninitializedObject` since accessing `Type.IsAbstract` could trigger a `MissingMetadataException` there. We got rid of that concept in NativeAOT because it cannot be reconciled with ILLink trimming. Rewrite the code to use `Type.IsAbstract`.

Saves 15 kB on BasicMinimalApis, which is nice. Also makes things faster since we avoid reading optional fields and rare flags.