Check for missing hierarchies when mapping to Roslyn projects

Add a common base type of IDE and diagnostics VS performance tests. 

Convert if-to-switch.

Analyzer A/B Test Info Bar

Update the code comment to explain why we need to check for null.

Fixes #19517.

In order to shows the "Analyzers" node under the "References" node in
Solution Explorer and show the correct sets of analyzer assemblies and
diagnostics, we need to map the node for the project to the actual
Roslyn `AbstractProject` instance. This is done by matching up various
properties of the `IVsHierarchy` for the node to the same properties on
the `IVsHierarchy` returned by `AbstractProject.Hierarchy`.

This code path was initially written this `Hierarchy` property would
never return null; its value was supplied by the project system.
However, Delayed Project Load (DPL) bypasses the project system until
the user expands the project node in Solution Explorer, and as a result
we may now encounter `AbstractProjects` where `Hierarchy` returns null.
Currently if we run across one of these we'll throw a
`NullReferenceException` which has the end effect of preventing the
user from expanding the project node in Solution Explorer, and may even
result in a crash.

This commit adds in the null check. If the `Hierarchy` property is null
then it can't be the project we're looking for.

Add back 'beta2' nuget moniker for official builds.

Fix case that was causing us to not offer completion after a case-pattern-label in a switch.

* dotnet/master: (217 commits)
  Ensure versions match.
  Skip test.
  Present feature to user as 'simplify default expression'.
  Add tests.
  Add fast paths for analysis.
  FOrmatting.
  Switch to DO / DO NOT style
  Meet the intention of CONTRIBUTING.md
  Copying content from WIKI
  Reorder checks.
  Move all option checking to a single location.
  Move version checking to single location.
  Simplify code.
  Update comment.
  Mark recently completed C# 7.1 features as such (#19483)
  Remove unnecessary casts.
  Formatting.
  Simplify code.
  Simplify code.
  Formatting.
  ...

Introduce a simplifier for "default(T) => default" expressions.

Remove code for generating constructors from class-decl, it will be provided by another refactoring.

Fix instrumentation for methods that have spans in multiple files.

Adds internal Compiler.CommonWithMetadataImportOptions and removes SymUnmanagedExtensions

Use file path to hook up analyzers nodes in Solution Explorer

Delete the PR branch if nothing to merge

The intent of CONTRIBUTING.md is to provide guidelines for contributing to a
repository.  Ours was a mix of guidelines, best practices and developer
workflow.  The other aspects are already covered in our other contributing
documents.

Changing CONTRIBUTING.md to be guideline focused.