This moves a few of the patches from sourcebuild into the Roslyn source proper.
We now condition on DotNetBuildFromSource in some of our imports, and ensure
that we're not targeting netframework when building for it. We also add a sln
for SourceBuild, SourceBuild.sln, which includes the minimum projects that it
needs.

…Regex

…Regex

Merging master into features/embeddedRegex

…ground thread

…Service

We take the analyzer references provided from VSIX extensions and create
a map of AnalyzerReference.Id -> AnalyzerReference, which is done by
cracking the metadata and getting the .NET assembly identity for them.
We were doing this on the UI during package load, which really isn't
necessary.

…exers

…izedObject

This is legacy localization code. Not needed anymore.

* the flaky-ness came from the fact that solution crawler has multiple async event queue to delay processing work (such as semantic prapagation event queue).

having multiple event queue is fine if test just wants to see all works queued have been processed since we can use waiter to wait for everything to finish.

but it is problem if we want to see order of work that got processed since event queue such as semantic prapagation queue can change order of things processed due to cancellation. (work A is being processed and semantic propagation want to cancel work A and mark it as semantic changed and re-enqueue the work)

so we want to wait for semantic event queue to finish processing but current big hammer approach of waiter (wait for everything in solution crawler to finish) doesn't let one to do some part of it to be waited.

added ability to the waiter to wait for parts of work to be done.

and now test waits for semantic event queue to finish before running analyzers.

* addressed PR feedbacks

* more PR feedbacks

* PR feedback

* PR feedbacks

* removed unnecessary export provider

Remove dead NuGet code

Use 7.2 language features in Roslyn

"Generate Equals" should ignore indexers and setter-only properties

Fix in optimizer to always respect refkind of vararg arguments

Merged on behalf of @Neme12. Thanks

…blishing

PublishData for AsyncCompletion

* __arglist cannot have an argument passed by 'in' or 'out'

* Fix build break

* PR Comments

Allowing multiple exposed line spans in VS style options preview

Implicit framework references were disabled at the time we moved over to
the new SDK due to some remanants of project.json in our build. Those
have all been cleaned up now and hence moving us over to using implicit
framework references.

Not having this enabled meant the API surface we were targeting in our
projects was not actually what was available in the project target
framework. Instead it was the intersection of the target framework and
the set of NuGet packages we reference. This meant that code which
should work simply wasn't due to the API surface not being correct.

Merging on behalf of @Neme12. Thanks!