I believe this fixes #52698,
although I don't have a dump for that issue so I'm having to deduce
what I think happened based on the stack alone. Unfortunately the stack
also has some frames inlined so there's a bit more guessing than usual:

The stack shows that we ended up in Comparer.Compare and it threw; the
only exception that it throws directly if the thing being compared
isn't IComparable. There aren't many uses of ImmutableSortedDictionary
in Roslyn, but the one specific one that makes sense to be under
GetDocumentState is the one we have inside our TextDocumentStates type.
And that one is one where we normally pass in a custom comparer to pass
in DocumentIds, so it seems logical that we somehow ended up with
a map that didn't have our comparer.

It turns out the Empty member we had didn't specify one, so if we
started with the Empty member and then added stuff from there, we'd end
up with such a map without a comparer. Why didn't we see this right
away? Because it turns out we only have a few uses of .Empty, most
of which are with source generated files. And even then we often are
creating the source generated collections with something else.

…-potentially-having-no-comparer

Always make sure we have the right comparer for DocumentIds

If we are processing edits through TryApplyChanges for closed files,
we'll make a series of changes in rapid succession: we'll have one
change that may open the file in an invisible editor, then we'll
edit the file, and then close again. The Solution object would then
be holding onto the old Compilation with several changes queued, which
would root the intermediate states that still pointed the editor
contents and their change lists which can be large. Once a Compilation
was requested for this new file, we'd process all the changes and
free the memory, but if the file is closed we might not get to that
right away.

…s-compilation-steps

Don't hold onto some SourceTexts if multiple changes happen

…ase/dev16.11-vs-deps

Merge release/dev16.11 to release/dev16.11-vs-deps

Keep OptProf input indefinitely

…ase/dev16.11-vs-deps

Merge release/dev16.11 to release/dev16.11-vs-deps