This will be the replacement for CompilerRequestHandler in the portable
layer

Turns out we do need a shared DLL project.  A lot of types that need to
be shared between the servers.

Just not a lot that we can share here.  Moving to separate projects for
now.  Will recreate a shared piece later if it makes sense.

Move the majority of the build logic to a shared project.

This doesn't completely work yet because ServerCore can't reference this
(it's a 4.5 portable project).  I need to remove the dependency on it
here and move the dependency out to VBCSCompiler.

Still a lot of BTODO items to clean up but can at least do a validation
build at this point.

Build CoreCLR projects into their own directory

Remove unused usings.

Cleanup of 'fix all'.

Simplify delegate invocation.

Fail the CIBuild if anything fails

Provide a sync path for 'AdditionalText' to prevent thread explosions.

Make note, that the Copy task throws on Mac/Linux, due to blinding
p/invoking into Windows APIs, so we explictly do not turn it on.

Fixes: #6423

Handle missing property explicitly

csi x-plat fixes

We used to continue on if we failed to build and/or copy the toolset.
We now fail, which will avoid issues covered in https://github.com/dotnet/roslyn/pull/6582.

Greatly improve performance of Object Browser first open

Fix VBSCompiler path in Toolset.sln

Object Browser was forcing compilations to be realized just to compute the assembly identity for each of a compilation's metadata references. With this change, the Object Browser will use the metadata references from the project and call the same utility function that is used to read assembly identity for analyzer assemblies.

Merge remote-tracking branch 'upstream/stabilization' into integration

Missed this sln file in the recent code reorganization.