Skip test for bug #6866

Audit usages of Compilation.References

Mitigate OutOfMemory when clearing large output buffers

Most callers are probably interested in
`Compilation.ScriptCompilationInfo.PreviousScriptCompilation` as well.
Behavioral changes expected in QuickInfo (fix crash) and SymbolFinder
(offer additional CodeFixes).

Fixes #6616 and #6657.

Protect against accidental merges to master

We remove all spans from the projection buffer *before* updating the
subject buffers so that composite/merged changes to the projection buffer
don't have to be computed.  This seems to alleviate the problem we were
having with OOMs when clearing large output buffers (which did not,
themselves, trigger OOMs).

We conjecture that this will also eliminate the race described in #6600
because the output buffer deletion should no longer affect the projection
buffer.  However, if the race was between input and clearing, then it may
still be an issue (though it's not clear how, since both edit the buffer
on the UI thread).