… completion

This change moves the MSBuildWorkspace tests to a separate project and introduces infrastructure to get them
running. Essentially, each test is marked with a `ConditionalFact` with `VisualStudioMSBuildInstalled` as the
skip condition. That skip condition uses the VS 2017 setup APIs to locate installed instances of VS and finds
one that contains the Roslyn Compiler package. Then, it sets up an AssemblyResolve event on the current AppDomain
to resolve MSBuild locations from VS itself. This ensures that MSBuild can locate a toolset containing the
necessary props and targets to handle the projects in our tests.

…stalled

…ing feature

* Produce errors on invalid pdbpath supplied to compiler

* More tests

While implementing the tests for the ref-reassignment test plan (dotnet#22466)
I found a bug in the handling of the ref-escape scope of a ref local in foreach. That bug is fixed
and the rest of the items in the test plan should have a test.

Fixes dotnet#22466

Fixing async keyword highlighting on local functions

Adding missing completion after 'ref'

….x-to-master

Merge dev15.7.x to master

Fixes dotnet#17261

 Not offering UseAutoProperty for volatile fields & when field used with ref

…en there is an unexpected difference. (dotnet#25296)

Branchless codegen for lifted nullable operators

Delete VisualStudioSetup.Next

* moving methods around to clean things up.

* more clean up

* added comment

* more code re-arrange

* revert some clean up back since that requires some changes I dont want to do for clean up

…atements.

Place '#pragma warning restore' in better location with multi-line statements