Other call sites appear to have been adapted already.
Fixes https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?_a=edit&id=266237

* Improve path completion providers

* Make GAC completion faster and cancellable

* Move FileSystemCompletionHelper.cs

* Move LoadDirectiveCompletionProviderTests.cs

* PR feedback and fix completion for 'X:' paths

Fix crash in AddUsing when changing casing of a typename.

Fix a small memory leak related to batching project loads

Fix configurations so that project will load in 15.3

Use a wait-free fast path in AsyncLazy<T>.GetValueAsync

Do a better job preserving blank lines when we convert code to using an object initializer.

Do not allow 'add nuget reference' for projects that we fail calling into Nuget for.

This PR updates all mentions of OSX/OS X to macOS in the README.

Fixes https://github.com/dotnet/project-system/issues/1918.

The language service implements `IVsSolutionLoadEvents.OnBeforeLoadProjectBatch` and `IVsSolutionLoadEvents.OnAfterLoadProjectBatch` to support asynchronous solution loads. While a batch is in progress we try to avoid pushing added projects to the workspace and instead store them in a list. When the batch is complete we may push all of those projects to the workspace at once.

The current implementation has a memory leak. Project may be added outside of a pair of `OnBeforeLoadProjectBatch`/`OnAfterLoadProjectBatch` calls, and it is possible for projects to be added without those methods ever being called. However, during a solution load adding a project always causes us to put it in the list of batched projects. Since we only clear the list on calls to `OnBeforeLoadProjectBatch` or `OnAfterLoadProjectBatch` the user could close the project or even the whole solution and we could still be holding on to one or more projects and all of their associated memory.

The fix here is to add a `bool` to track whether or not we're batching projects, and only put an added project in the list if we are.

Disable scanning for designer attributes in CPS projects.

Be resilient to having a project that doesn't know about ArgumentNullException or ArgumentException

Fixes #19171
Fixes #19174
Fixes #19173

TypingHelpDirectiveWorks()   - https://github.com/dotnet/roslyn/issues/19232
WorkspacesNetCore.ProjectReference() -  https://github.com/dotnet/roslyn/issues/19231

Ensure that we get a well formed child IOperation tree even for bound…

Add type inference support for tuples.

Enable multiline regex mode for "test <whatever> please"

Include DSRN in toolset package

Merge dev15.3-preview1 to master