Merge latest bits from dev16.0.x to features/async-streams

update Optimization data to 2.11.0-beta1-63312-03

* Calculate CompilerArtifacts in a target

Calculating the set of files in evaluation phase is incorrect, since it depends on files of dependent projects to be copied to output directory.

Don't mark Microsoft.CodeAnalysis.Elfie as a private package

This is a package we depend on via normal means, and expect it to
be referenced in our NuGet packages as they are today.

Merge dev15.9.x to dev16.0.x

Revert "Fix #30091 throwing exceptions"

This reverts commit b3ac2a6f.

Merge dev15.9.x to dev16.0.x

The general pattern when registering code fixes seems to be
either passing diagnostics[0] or diagnostics.First().

RemoveUnusedVariable seems to be the only outlier in this case,
doing .Single().

Having multiple diagnostics on a span would cause the code fix
to throw an exception

Add support to save current tools options into an .editorconfig file

default to vsn if NGenApplication is not set

Add test for Span not allowed as lhs of ??=

This ports https://github.com/dotnet/roslyn/pull/28472, that went into the `editorconfig-ide` feature branch, into `dev16.0.x` branch with following additional refactorings/enhancements:

1. Generated .editorconfig file has all localized string as comments.
2. No hardcoding of individual options in the editorconfig file generator. Instead, each `EditorConfigStorageLocation` now requires to provide a delegate to convert a given option value into the .editorconfig string representation (basically complimentary of the parsing delegate that was already required).
3. Introduce concept of `OptionGroup` to enable grouping options in the same feature (such as CodeStyleOptions, CSharpCodeStyleOptions, FormattingOptions, CSharpFormattingOptions, etc.) into sub-features or groups (such as "this. preferences", "expression-level preferences", etc.). This enables the editorconfig generation to group the related options, without explicit knowledge of each option group.
4. Expose `AllOptions` from each option feature (such as CodeStyleOptions, CSharpCodeStyleOptions, FormattingOptions, CSharpFormattingOptions, etc.), so that the corresponding `IOptionProvider` always stays in sync with implemented options.