* 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.

fix for language code capitalization

This reverts commit 6b22ff18.

Relates to #375

Co-Authored-By: NSam Harwell <sam@tunnelvisionlabs.com>

Co-Authored-By: NFred Silberberg <fred@silberberg.xyz>

Fixes #39731.

- Declaration of classes, structures and enums within variant interfaces is disallowed.
- Nested classes synthesized by the compiler don't have variant emitted type parameters.
- Local functions and lambdas used within a variant interface always have display class.

* Implement lambda discard parameters

* Remove duplicate tests

* Factor logic to recognize underscore tokens

* Align symbol display with other discards

* Distinguish discards in QuickInfo

* Add IsDiscard property instead of IDiscardSymbol

* Adjust IDE logic to use IsDiscard

* Avoid large tuple return

* Add test for ref/out discard parameters

* Address PR feedback and test plan ideas

* Allow in expression trees

* Support general discard parameters

* Add IDE tests

* Emit with unspeakable name

* Remove unused error code

* Only support lambdas

* Fix tests

* Address some PR feedback

* Address remaining feedback

* Address more feedback

* Fix conflict

* Update PublicAPI.Unshipped.txt

* Update PublicAPI.Unshipped.txt

* Add ChangeSignature tests

* Add SymbolCompletion tests

* Add InlineRename test

* Typo

This change also separates implementation of ISymbol interfaces from internal symbols in C# compiler.