Rewrite PopulateSwitch diagnostic and fixes.  Fix bug in generator that causes crashes when generating a default case for VB.NET switches.

Also add support for completion item filtering.

user can either explicitly turn it back on or re-start VS to re-enable the full solution analysis.

Now, Add-Using will use the indices if they have been built.  Or not search
those projects/metadata if they're not available.

Note: this doesn't change any of the searching of the current project.  That
works the same way it always has.  This is only for searching unreleated projects
and metadata dlls.

Cleaning up some usings/Imports at the same time

Extract classes into their own files.  Make the max number of results for add-using/fully-qualify '3'.

The recommendation tests can be run in parallel but were in an assembly which
had many other tests that could not. Splitting out into a separate assembly
which is designed for parallel runs.

This change does two actions to our public API files:

- Resets them to the versions from 1.0.0 RTM
- Splits them into the shipped and unshipped files

We execute the analyzer driver's initialization and core analysis tasks on a background thread. Any exceptions from the driver itself (not the analyzer callbacks, we already report diagnostics for those) were getting swallowed and analyzer execution also skipped silently. See #2980 (comment) for an example.

We now report an exception diagnostic for analyzer driver crash (with the complete exception trace), so that the user knows what happened and we can diagnose the bug when the issue is reported.

Fixes #3005

Several options in the Features layer were still defined as static fields attributed with ExportOption, which is located in Workspaces.Desktop. Portable MEF does not support static fields for exports so it's necessary to implement IOptionsProviders for each set of options.

Changes include:

1. Enhance CompilationWithAnalyzers to allow computing analyzer diagnostics for a specific tree/span within a compilation and/or for a subset of analyzers. Implementation ensures no duplicate analysis by tracking partial analysis state and caching the reported analyzer diagnostics.

2. Overview of the new APIs added to CompilationWithAnalyzers:
    1. GetAnalyzerSyntaxDiagnostics(tree, analyzers, ct)
        1. Analogous to SyntaxTree.GetDiagnostics(ct)
        2. Gets analyzer diagnostics reported by executing syntax tree actions on the given tree.
    2. GetAnalyzerSemanticDiagnostics(semanticModel, spanOpt, analyzers, ct)
        1. Analogous to SemanticModel.GetDiagnostics(spanOpt, ct)
        2. Gets analyzer diagnostics reported by executing rest of the non-compilation actions on the given tree span.
    3. GetAnalyzerCompilationDiagnostics(analyzers, ct)
        1. Gets rest of the analyzer diagnostics which are reported by either of the following means:
            1. Compilation actions (and compilation end actions)
            2. Non-compilation actions reporting diagnostics on different tree: Executing a symbol action on a symbol definition in a tree, can report diagnostic on its partial definition in some other tree.

3. Simplify IDE analyzer driver by switching it to using the new CompilationWithAnalyzers APIs for analyzer diagnostic computation. Both the IDE and compiler drivers now use the compilation event queue model for driving analysis.

"Roslyn" -> "Microsoft" or "Microsoft.CodeAnalysis"

At the moment, these aren't used by any completion providers in the features layer. Until we decide that this is ready to support cross-platform, let's keep them in the editor features layer closer to the providers that actually use them.

- apply feedback: unify C# and VB