Properly 'go to implementation' when there are abstract overrides in the hierarchy

Allow 'none' to be the severity for options in .editorconfig

* Update toolset compilers to 2.3.0-beta2-61716-09. Use LangVersion=7.1
* Revert LangVersion for VB. Follow-up issue #19610

Relates to #19038, #16195

Support serializing longs to/from the local registry

Don't go to the threadpool to compute data that is called synchronously by the editor.

Bump RoslynNuGetMoniker to beta3

Decided to go with procdump for getting our dumps.  While it's cleaner
and easier to create the dumps with a simple PInvoke call it's not a
complete solution.  The MiniDumpWriteDump API will always write the dump
in the architecture of the process calling the API.  Hence it would
create them for the architecture of RunTests, not the architecture of
the process being dumped.

In order to make it work I was going to have to buil two versions of
RunTests, or a simpler dumping program.  At that point it's just easier
to use ProcDump.  Even though I rather dislike an explicit download step
at the start of our CI runs.  Would be much nicer if sysinternal was
offered as NuGet package.

But the benefit here is clear so going forward with this.

This contains two changes to our test runner:

1. Adds a -timeout value.  When that is exceeded dump files will be
written for the remaining processes and Runtests will exit with an
error.
1. Adds a sample timeout for our VSI tests.

We don't support serializing uints. We could add support but it's a
pain and there's no specific reason to need uint here.

The underlying issue has been fixed, so we can renable this check.

Fixes dotnet/roslyn#19629.

Show a description when suggesting names

log info for ab testing

add F# language name and error prefix

Merge dev15.3-preview2 to master

Fixes #14295

Fixes https://devdiv.visualstudio.com/DefaultCollection/DevDiv/_workitems?_a=edit&id=388743

Fixes #18948

See #19682

Fixes #19675

* Fixes crash in `is` operator used with tuples
Fixes #19310

* Matching an expression of type `byte?` with a pattern of type `int` should not be allowed
* Add documentation for the breaking change matching `int?` against the pattern `long x`
Fixes #19151

* Unify subsumption implementations
Fixes #19273

* Greatly improved code gen for value type pattern match
Fixes #16129
Fixes #18944
Fixes #14252

* Add test confirming that #19122 is fixed.
Fixes #19122

Fix for deconstruction-assignment to write-only property.