Fix UMEntryThunkCache::GetUMEntryThunk

[janvorli]

The function was initializing UMThunkMarshInfo allocated from Stub heap
without using the ExecutableWriterHolder. That causes a crash when a
hosting application calls coreclr_create_delegate.
This was discovered in .NET 6 Preview 6 when running a xamarin app that
uses a special host.

This code path is exercised only by coreclr_create_delegate.

Close #55773

[jkotas]

There is nothing executable in UMThunkMarshInfo . Why are we allocating it on executable heap in the first place?

[janvorli]

Why are we allocating it on executable heap in the first place

I think the reason was that before my changes, the UMThunkMarshInfo was being allocated from two heaps. From regular C heap and from the thunk heap in Module. I needed to unify it so that at some place I don't need to somehow detect which one it was when a write to it was needed. I am not sure if it was even reasonable to detect which one it was. And somehow moving the one in the Module to C heap was complicated, IIRC.
The thing is that I have a local change from before that moved this whole UMEntryThunk related stuff to the new way of splitting executable and RW data into separate pages that I want to add post 6. So I just kept it simple.

[janvorli]

/backport to release/6.0-preview7

[github-actions]

Started backporting to release/6.0-preview7: https://github.com/dotnet/runtime/actions/runs/1038308232