…ngCompilationTracker

This is just a simple conversion of the field to a property.

In this particular code path, we were passing null for the frozen
source generated document state when we could have passed it along.
The original logic here was frozen source generated documents only
normally appear when somebody calls
ITextSnapshot.GetOpenDocumentInCurrentContextWithChanges, since we need
to ensure that there is a generated document with that exact snapshot
for the feature to work. We should never see that in a "primary"
solution in any way. However, solution sync uses CurrentSolution as
a cache for the previously synced complete solution, which meant
we'd still end up calling this anyways.

Although we could (and probably should) avoid caching these special
solutions entirely, this avoids any surprises here and makes things
behave like normal, and avoids odd cases where the CompilationTrackers
are out of sync with this field.

This partially addresses dotnet#57082

When we are synchronizing solutions, the previous assumption in this
code was incorrect -- we absolutely can end up with a frozen generated
document in CurrentSolution, since the remote workspace caches things
there. We're going to just run with that and consider that normal, so
we'll fix up the code to support it cleanly.

Fixes dotnet#57082
Fixes dotnet#56998