Extend Linux CI support

Added couple tests for ?. operator for cases missed in optimizations.

Test-only change

The tests arep assing, but codegen is not great.

Use msbuild variables for assembly versions of Immutable and Metadata references

This reverts commit 94b22b7c.

The issue was the "DirectCast(Nothing, CompileResult)" (later removed by
commit 90bb89f0).  When compiling without
optimizations, this returns a structure initialized to its default value.
When compiling with optimizations, this throws NullReferenceException.
I've filed bug #2094 for the inconsistency.

Introduce msbuild variables for versions of Immutable and Metadata nuget packages

Retry with System.Core.dll on ERR_NoSuchMemberOrExtension.

Suppress expansion of void* rows

...because they would have type ```void```, which can't be instantiated.

Avoid a hang in CompileWithRetry...

Don't attempt to retry if we're asked to load the same assembly identity more than once.  Adding the same reference won't help anything, and we will almost certainly never terminate the retry loop.

Add support for "dynamic" in the result provider

This is the first of a sequence of changes.  We associate additional
information with each type that comes from the expression compiler.  For
now, it is the bool[] from the DynamicAttribute on the return type of a
method synthesized by the expression compiler.  (The bool[] is packed into
a byte[] for generality and compactness.)  Then, as we traverse the type
in the type name formatter, we consume the flags to determine which
occurrences of "object" should be replaced by "dynamic".

We don't show "dynamic" everywhere we could (e.g. in base type names in
disambiguators), but I believe we show "dynamic" everywhere the legacy EE
did.

TODO: The expression compiler doesn't call the new API to pass the
appropriate dynamic flags (API pending).

TODO: The expression compiler doesn't consume dynamic flags on
pseudo-locals (API pending).

Remove Win10 check from assembly reference resolution...

Ignore enums in OverrideMethodsOnComparable. fixes #1994 

Change uses of FailFast.Assert to Debug.Assert.

In Windows 8 (and 8.1), runtime winmds may contain assembly refs pointing
to other runtime winmds (for example, types defined in Windows.UI.Xaml
point to Windows.UI).  However, when the EE sees these assembly refs it
must redirect them back to the compile time contract assembly (Windows),
because the Compilation used by the EE only references the contract Assembly,
not the various runtime Assemblies.  Previously, we only did this redirection
for Windows 10.  Non-EE cases should only see the contract assembly at
compile time, so this change will not affect them.

Ignore enums in OverrideMethodsOnComparable analyzer and codefixer.
fixes #1994

Add a test for generated IL for a?.b == c in control-flow context.

Removed a duplicated row for the metadata token TypeRef

Ensure that SemanticModel binds with an IncrementalBinder if result of the binding is added to the cache.

update comment and add unit test for VS workspace diagnostic analyzer provider service

Consume new async light bulb APIs from VS platform. 

This makes the light bulb preview pane render asynchronously so that expensive previews don't block the UI thread.

Ensure that SemanticModel binds with an IncrementalBinder if result of the binding is added to the cache.

Fixes #1504.

Add handling of ArrowExpressionClauseSyntax nodes to LocalBinderFactory.

In cases where primary document from which the rename tracking fix was triggered doesn't have any changes (aside from the rename which is already in the buffer), we would end up offering no preview. This change ensures that we will go on and return preview for some other document that did have changes in such cases.

Fix perf issue that was causing the tagger that's used in preview pane to unnecessarily peg CPU on UI thread. We were unnecessary cycling through all documents in a project when we only needed to process a single document.

Consume new async light bulb APIs. This makes the light bulb preview pane render asynchronously so that expensive previews don't block the UI thread.

Upgrade the BCL NuGet packages to RC versions.

Upgrades System.Collections.Immutable to 1.1.34-rc and System.Reflection.Metadata to 1.0.19-rc.