Clean up and fix management of ExplicitThis in the CoreCLR native runtime

[trylek]

According to the PR comments in

#73341

CoreCLR runtime handling of explicit this in function pointer signatures is incomplete and inconsistent
and needs improvements and fixes. This issue tracks this work that is expected to happen in .NET 8 timeframe.

Thanks

Tomas

/cc @jkotas @davidwrighton - please feel free to add more details or clarifications, my expertise here is still quite limited.

[jkotas]

cc @steveharter This is related to function pointers support in reflection

[jkotas]

#69273