The TPA now considers the output directory as a trusted place for
loading assemblies. To the point it will prefer certain DLLs in the
output directory over equal ones that ship with the runtime.

This broke a number of VB tests here because we unconditionally
referenced the desktop version of Microsoft.VisualBasic. This got copied
to the output directory, included in the TPA and hence loaded during
test execution. This breaks tests because they require the .NET Core
version of Microsoft.VisualBasic.

Conditioned the reference to be desktop only so it's not present on our
.NET Core builds

Decided to remove the property based approach to specifying a target
framework to just specifying `netcoreapp3.1` directly. The reason for
this is the following:

The advantage of the property is it makes it "easy" to change to a new
target framework in the future. That benefit is actually pretty minimal.
A simple find and replace operation is **extremely** effective in our
code base (it's less key strokes than this message). Hence the benefit
is minimal.

The downside of the property is that our code doesn't look like customer
code. Or rather it diverges from the practices that we publish. In
general I prefer to keep our code as standard as possible unless there
is a good reason to deviate. There just doesn't seem to be one here.

https://github.com/microsoft/msbuild/issues/4806

Change Roslyn to target netcoreapp3.1 when building .NET Core assets.

Previously the code targetted a mix of netcoreapp2.1 and netcoreapp3.0.
The mix is due to default interfaces only being supported on
netcoreapp3.0 and hence our testing needed to use that. Yet at the same
time we were required to ship the compiler in SDKS that targetted
netcoreapp2.1. Now we can universally target netcoreapp3.1.

Properly generate suppression attributes (including usings/imports)

Properly exclude changes to unchangeable documents from being applied

Co-Authored-By: NManish Vasani <mavasani@microsoft.com>

Co-Authored-By: NManish Vasani <mavasani@microsoft.com>

Co-Authored-By: NManish Vasani <mavasani@microsoft.com>

Co-Authored-By: NManish Vasani <mavasani@microsoft.com>

Code block callbacks

Enable nullable reference types for Microsoft.Build.Tasks.CodeAnalysis

Optimize compilation tracker for solution close

Remove dependency on TPA list for testing

Fixes #40629

Update code style analyzer to 3.5.0-beta3-20078-04

Update Code Style links.

Check cancellation in GeneratedCodeTokenWalker