Fixes EnC slot mapping for certain kinds of synthesized variables lifted to state machine fields

[tmat]

Fixes https://github.com/dotnet/roslyn/issues/63294

In C#, we were not correctly mapping slots for variables holding on closures that were lifted to state machine fields. In VB, we were accidentally mapping these correctly but were not mapping lifted variables that store values of With statements.

[tmat]

@davidwengier @cston @jcouv PTAL