Adds a new "Preview" language version for C# that means the latest
supported language version + the preview features available in the
compiler. Also changes the "Default" (unspecified) language version to
mean Preview, but provides a warning when preview features are used.

Make the flow analysis based unused value/parameters analyzer more conservative

don't retain fault result in task center if nuget info download failed.

Classify keywords in ForEachVariableStatements as ControlKeywords

If a solution is closed and reopened, still recreate a SolutionId

Do not report unused expression value (IDE0058) for expression body

Updated version to 3.0.0 beta 4

to retain fault result in task center, we need to provide detail info UI so that user can click the fault task left in the task center, but right now we don't have one and we don't want to just show exception message in message box.

we could think of using RetainAndNotifyFault so that we show exception message in the task center tip, but that would be annoying since most likely it is info user didn't ask for it.

we can add UI later if user ever ask for it. for now, taking simple approach of just removing it from task center once download is done in any way (success, cancelled, failed)

Re-enable csharp symbol tests on mono

Merge dev16.0-preview3 to master

Fixes #32942

Prior implementation looked for specific unhandled operation tree shapes and was agressive, leading to false positives. New implementation handles only specific operation tree shapes and is much more conservative.

Fixes #32946
Fixes #32924

Avoid unnecessary thread-local variables

Localize regex tests

Fixes #32973

Unused value assignments will not be flagged in such operation blocks. Unused parameter reporting falls back to checking presence of any read references to parameters, regardless of flow feasibility of that code path.

Fixes #32959

Fixes to compiler generated code script

Fixes #32923

Fix a bad merge that left a merge conflict marker in a user visible string.

Makes the following changes to the script which generates our compiler
code files:
1. No longer depend on RID. That was an accident of history and it
caused VS errors when the script was run with VS open. Now just use
"dotnet run" on the project.
1. Put the temporary files under the artifacts directory so they are
included in our published artifacts on error

Merge dev16.0-preview3 to master

Fix assertion in UsePatternMatching analyzer

Fix rootnamespce property name we received from project-system

fix some out-of-sync file names

update mono-devel in dockerfile, and don't cache later commands

* Classify C# 8 ControlKeywords

* Add unit test for C#8 control keyword classification