[interp] Kill InterpFrame.ex (mono/mono#16666)

[interp] Kill InterpFrame.ex (mono/mono#16666)

* [interp] Reduce unnecessary uses of frame->ex

* [interp] Remove ex from InterpFrame

We save stack space and move the exception where it belongs, in ThreadContext. It was confusing what frame->ex really meant, it was probably not used / initialized properly. We move it now to ThreadContext where it represents the current exception being thrown during EH (when we have a resume state set).

Commit migrated from https://github.com/mono/mono/commit/f0dabc6618931ccd4c544657ed1a7f3d3d803c25