Since ImmutableDictionary and ImmutableSortedDictionary are both
implemented via trees; you can think of an ImmutableDictionary as just
being a sorted dictionary where the underlying key is the hash int.
Since our hash is just a hash of a GUID, we can compare by the GUID,
switch to a ImmutableSortedDictionary, and then avoid another sort
when we're computing checksums.

We previously were using ImmutableArrays, so if you had a large number
of documents (say thousands) we'd be creating huge numbers of huge
arrays over and over again. For this pattern, ImmutableList is more
appropriate.

move designer attribute back to document level scanner but made it to use keep alive connection rather than creating new connection every time. also kept some optimization such as checking cps projects or not reporting as much as we can. (#22248)

* IDE Changes

* tests

* PR comments

this basically prevent symbol info indexer from creating/calculating same checksum again and again for every documents in the solution.

this should make it to do it only once per project rather than document, and only update it when there is a change in the project.

Add MonoDevelopGtk synchronization context as consumption of Roslyn in VS for Mac uses this context.

We don't need the split anymore now that we're only supporting Dev15.

Unfortunately this required the introduction of a NuGet package that
has a namespace "Workspace" to our VS project, which meant I had to
disambiguate Workspace wherever it's used in that layer. I considered
doing an extern alias instead to disambiguate, but applying that to a
ProjectReference isn't supported. That's tracked by NuGet/Home#4989.

added ability to run OOP in 64bit. currently it is not enabled. but it can be enabled explicitly through options or remotely through a/b testing.

a/b testing is not enabled yet.

* Load DSRN from alternate path if set

* Fix error reporting

This primarily moves us to the 2.2.0 version of xunit. In order to make
that transition though a number of other changes needed to happen as
well:

1. Needed to move our desktop target to 4.6.1. This should've been
required some time ago but we were essentially depending on bugs in the
SDK / NuGet.
1. Remove the DeployCoreClrTestRuntime project. This entire idea
depended on having the xunit.console.netcore package. This has since
been deleted and doesn't work with the 2.0 SDK.
1. Remove the deploy compiler tools project. Can just use dotnet exec
now and that's much more efficient.
1. Resolve a new ambiguity with KeyValuePair. A helper of ours now
exists in netcoreapp2.0.

I fully expect a number of Jenkins legs to fail with this change: build
correctness, coreclr and ubuntu. But I want to validate our other legs
and am still trackind down answers needed to fix the others.

* Add and implement IScriptEnvironmentService

* Remove unnecessary export

Fixes #17872

Fixes #11490

with set SERVICEHUBTRACELEVEL=All set, vs should put more logs in %temp%\servicehub\logs

while doing this, already found an issue where JsonRpc would throw bad RPC head exception. working with ilya to find out how that can happen.