Handling of neutral culture in AssemblyIdentity

Add System.IO.FileSystem to Microsoft.DiaSymReader.PortablePdb packages.config

Handle corrupted default values in metadata

Move all scripting tests to the new API and tweak the API to simplify some scenarios

Address a performance regression in the v2 analyzer driver

1) Perf traces show large GC activity during diagnostic analysis. Switched a ConditionalWeakTable added in the v2 driver change from being keyed on SyntaxReference to being keyed on Compilation, with the value being a dictionary keyed on SyntaxReference's Location. Also added cache cleanup logic to ensure entries from this dictionary get cleared as declarations are analyzed across all analyzers.
2) With the v2 driver refactoring, we stopped using the descriptor cache for analyzers in the IDE. This change re-introduces the cache and also adds logic to avoid realizing all the compiler diagnostic descriptors during diagnostic analysis.

The native compiler handled corrupted default parameter values by
substituting in default(T).  It's unclear if this was intentional
behavior in the compiler or not.

Either way though obfuscators took advantage of this behavior and at
least one prominent one will corrupt default parameter values when the
value is null.  Enough prominent libraries have shipped using such
obfuscators that it is a blocker to upgrading.  Hence we need to emulate
the native compiler behavior here.

This change is a bit large because I needed to update the test resources
with a corrupted DLL in order to test out the changes.

closes #4196

Enable selection of Mef components based on TextViewRole...

We need a way to provide alternate completion in the Interactive Window.
Previously, we weren't taking TextViewRole into consideration when
selecting completion providers.  This change allows a service to opt in to
considering TextViewRole by passing OrderableLanguageAndRoleMetadata to
SelectMatchingExtensionValues.

This change also includes some cleanup of test helpers and removal of extra
TextViewRole attributes that weren't doing anything meaningful.

Move all IInteractiveWindowOperations implementations to UIThreadOnly

Be less aggressive about parsing out accessors in an accessor list.

Compilers.sln and Workspaces.sln shouldn't reference portable TestUtilities

Don't infer 'Object?' in an null coalescing expression.

Fixes #4483

Merge branch 'completionInIncompleteProperty' of https://github.com/CyrusNajmabadi/roslyn into completionInIncompleteProperty

Ignore DkmClrNcContainerModuleInstance instances

Fixes #4840.

Unify code between the C# and VB type inferrers.

Add default ToString to SyntaxTree

pushed down GetRelatedDocumentId API down to solution.

Add padding around the lightbulb diff preview

Remove Portable.FX45/project.json, clean up list of global packages

Add EquivalenceKey for suppression code actions to ensure that duplicate equivalent suppression fixes are not shown.

Fixes #3311 

Conflicts:
	src/Test/Utilities/Runtime.FX46/TestRuntime.FX46.csproj.nuget.targets
	src/Test/Utilities/Runtime.FX46/TestRuntime.FX46.nuget.targets

Fixup tests (which now show better error recovery).