* Clean up string resources, use Arcade resx generator

* CodeLens resources


* Use NET20 instead of NETFX20

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.

* Move "is telemetry allowed" flag to DiagnosticAnalyzerInfoCache

* Clean up DiagnosticService

* Fixes

* Remove analyzer exception telemetry reporting.

* Remove reporting analyzer exceptions from SupportedDiagnostics.

The diagnostics will be reported when the compiler runs the analyzer on the compilation.

* Remove unused parameter

* Clean up analyzer telemetry reporting.

* Test fixes

* Feedback

Fixes #41207.

This reverts commit 6b22ff18.

Relates to #375

Respect filterTree and filterSpanWithinTree while collecting diagnostics for misplaced doc comments. (#39525)

Fixes #39315.

Move optimization around use of Nullable type in Boolean Expressions into LocalRewriter and eliminate invalid optimization. (#38802)

* Move optimization around use of Nullable type in Boolean Expressions into LocalRewriter and eliminate invalid optimization.

Fixes #38305.

The goal of optimization is to eliminate the need to deal with instances of Nullable(Of Boolean) type as early as possible, and, as a result, simplify evaluation of built-in OrElse/AndAlso operators by eliminating the need to use three-valued Boolean logic. The optimization is possible because when an entire Boolean Expression is evaluated to Null, that has the same effect as if result of evaluation was False. However, we do want to preserve the original order of evaluation, according to language rules.

* Fix an old bug masked by #38305.

* Fix an old bug masked by #38305.

* More fixes fo old bugs masked by #38305, plus optimizations that wouldn't be performed by compiler.

* PR feedback

Fixes #24737

Spelling fix

Fixes #38711.

Adjust inconsistent accessibility checks to properly handle protected types declared within interfaces. (#38451)

Fixes #38398.

Related to #38454.

These tests fail in the CI runs for #38417 which adds translations for various resources. The problem is the test compares localized text with the original English text; this worked fine until now because the localized text wasn't yet translated, and so just happened to be the same as the English.

The point of this change is to unblock #38417.