Only root infrastructure and dependencies for IDynamicInterfaceCastable when IDIC is implemented

[jkoritzinsky]

Implement a mechanism to conditionally root [RuntimeExport] methods based on the type corresponding to their feature being used. Use this mechanism to only root IDynamicInterfaceCastable logic when someone implements IDynamicInterfaceCastable and provide "default" implementations that should never be called in the bootstrapper when IDIC is unused.

Additionally, enhance the ScannedDevirtualizationManager to make two observations that we can optimize around:

• Note when IDynamicInterfaceCastable has any implementors
• If a [DynamicInterfaceCastableImplementation] type is rooted but IDynamicInterfaceCastable is not, the interfaces implemented by the attributed type still can have a known set of implementors.

Finally, adjust the "what type handle to use for casting" logic to only root a MaximallyConstructableType symbol when the target type is not IDynamicInterfaceCastable and when IDynamicInterfaceCastable is implemented in the compilation (based on DevirtualizationManager).

[jkoritzinsky]

I've adjusted the InterfaceDispatchCellNode to do the same optimization as the cast target symbol optimization and that has allowed much of this work to cleanly work.

With the current changes, we have 1.7kB from IDynamicCastableSupport (and 32 B from IDynamicInterfaceCastable) preserved even when IDynamicInterfaceCastable is not used. I'll see if I can adjust the body substitutions logic to allow us to stub that out when IDynamicInterfaceCastable is unused.

[jkoritzinsky]

I've added a commit to explicitly substitute away the entry-point to the logic that I couldn't get trimmed away so we can get most of the size savings now.

[MichalStrehovsky]

(Sorry for the slow reaction time, missed this PR went out of Draft)