For reasons that aren't entirely clear, when we create a preview of
a code fix, we remove and re-add documents that are being changed when
we produce the PreviewWorkspace. This process had an bug which meant
that the file path of an .editorconfig got dropped, so the resultant
.editorconfig document path was null. This later caused a crash if
the diagnostic engine, when processing the PreviewWorkspace's
new solution, tried asking for a Compilation, since the null path would
get passed to AnalyzerConfig.Parse(), and it would throw.

This was exposed by dotnet#45076; until then if you had a preview of just
an .editorconfig file, nothing would actually be analyzed and so
nothing asked for the Compilation. The crash is "fixed" by dotnet#44331 in
that asking for a Compilation no longer results in a call to
AnalyzerConfig.Parse(); you have to ask for a Compilation and then do
something with a semantic model. By luck, that doesn't happen in the
common repro, but more obscure things absolutely could still fail.

This is being tested by the integration test being added in dotnet#46639. We
don't really have a good unit tests here as far as I can tell, but even
then a unit test really doesn't validate any of the end-to-end here.

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1162464

Closes dotnet#44131

dotnet#45076 added support for reporting analyzer diagnostics in additional files. This PR addresses pending work from dotnet#44131 (comment) by adding similar support for editorconfig/globalconfig files in the project.

There are two primary public API additions