Move to .NET 5 Preview 8 SDK

* Don't offer to generate properties or fields in the global error class

* Cleanup

Add tests for nullability with IDE0007 diagnostic

Initial IDE test sweep for extended partial methods

Attach service to workspace disposal token.

* Allow record's `parameter_list` to be empty.

Related to #47091
https://github.com/dotnet/csharplang/blob/master/meetings/2020/LDM-2020-08-24.md#base-calls-on-parameterless-records
https://github.com/dotnet/csharplang/pull/3900

The Azure DevOps service is currently in the process of transitioning to
a new Mac pool. The machines in the new Mac pool are observably slower
than the ones in the old pool.

This difference in speed appears to be the cause of our recent
instability in the Mac jobs. Consider the following two jobs as a data
point:

- [Passed Build](https://dev.azure.com/dnceng/public/_build/results?buildId=818909&view=logs&j=1402d8c5-7e47-5bf4-467f-73e45e01bcce)
- [Failed Build](https://dev.azure.com/dnceng/public/_build/results?buildId=819817&view=logs&j=1402d8c5-7e47-5bf4-467f-73e45e01bcce)

At first I believed the source of the issue was a hang in the C#
semantic tests. Pretty much all our OSX failures came when executing
that test assembly. In a retent PR though I added a verbose log support
and discovered it was failing on the test
`AccessPropertyWithoutArguments`. That is a pretty basic test, no reason
I can see that it would result in a Mac only hang bug.

Decided to break down the runs into different sections and see how long
it took each build to get to that section

| Event | Passed Build Time | Failed Build Time |
| --- | --- | --- |
| Begin Real Build | 10 min | 12 min |
| Begin Running Tests | 30 min | 54 min |
| Begin C# semantic test | 39 min | 78 min |

It seems very likely that the passed run ran on the old Mac pool and the
failed run on the new Mac pool. The slow down lines up with what other
repositories are seeing.

As such I'm going to increase the timeout to account for this change. If
my analysis is wrong and it's a real hang it should continue to
reproduce after this change and I'll investigate further.

* Require partial method signatures to match

* Revert changes

* Fix tests

* Fix tests

* Avoid duplicate errors for tuples

* Fix tests

* Update numbering

* Add distinct error for nullability mismatch

* PR feedback

* Another test

* Fix test

* Add warning wave warning

* Update comments

* Remove stale comments

* PR feedback

* Fix test

* PR feedback

* Update comments in CopyTypeCustomModifiers()

Fix regression in SuppressMessageAttributeState.TargetSymbolResolver

The verbose logging option causes xUnit to output starting / finished
markers for each of the tests that it is executing. This will help us
track down the test that is hanging on Mac so frequently

* Remove test caching layer

The test caching layer is inactive today and unlikely to be revived in
its current form. Test caching is much more likely to come at the Helix
layer in the future. The cache code is just making the runner harder to
maintain at this point hence removing it for now.

Merge pull request #46884 from DevSnicket/33047-fix-msbuild-workspace-default-global-property-override

msbuild workspace global property defaults can be overridden

Supply completions for negative regex categories as well.

Harden DiagnosticResultSerializer diagnostic reader part

bugfix/msbuildworkspace duplicate analyzer references

* upstream/master: (114 commits)
  Remove langversion restriction for source generators. (#47714)
  Adjust disambiguation rules for type pattern in a switch expression. (#47756)
  Delete decommissioned benchview tool scripts (#47752)
  Emit conversions between native integers and pointers directly (#47708)
  Typeless expressions should contribute nullability to lambda return (#47581)
  Use a distinct diagnostic ID when an exhaustiveness report uses an unnamed enum value. (#47693)
  [master] Update dependencies from dotnet/arcade (#46586)
  Change `Location` of record's primary constructor to point to record's identifier. (#47715)
  Add public API test for extended partial methods (#47727)
  Rename in CheckValidNullableMethodOverride (#47718)
  Update docs
  Add more doc comments
  Add comments and doc comments for ExternalErrorDiagnosticUpdateSource
  Add documentation remarks for syntax kinds (#43646)
  Disable TestCancellation (#47725)
  Classify function pointer punctuation (#47668)
  Disable flaky optprof test
  Handle NotNullIfNotNull in delegate creation and overrides (#47572)
  Adjust QuickInfo on record BaseType syntax (#47656)
  Don't exclude events for NameOf context
  ...