Update Roslyn package references to latest available and do minor fixup

[alexgav]

Summary of the changes

• Update Roslyn package references to the latest in VS main
• Minor fixup due to ambiguous extension method Reverse() - there is now one in System.MemoryExtensions, so we need to force type we are invoking the method on to be IEnumerable to get the Linq reverse method we want.
• Roslyn GetDocumentSymbolAsync method now requires supportsVSExtensions parameter. Hardcoding to true for now until we plumb VSInternalClientCapabilities through init of the remote client (see Pass VSInternalClientCapabilies (or needed parts) in RemoteClientLSPInitializationOptions #11102)

[DustinCampbell]

Very glad that you separated this out from your other PR! This is definitely a separate concern from the completion work.
 
I added a comment that describes the changes needed to correctly pass the VSInternalClientCapabilities.SupportsVisualStudioExtensions from the client. Also, it looks like this change causes test failures. Those will need to be fixed up before I can sign off.

[alexgav]

Also, it looks like this change causes test failures. Those will need to be fixed up before I can sign off.

All appear to have been due to the new IRazorMappingService that's in Roslyn but not yet in Razor. Returning null rather that throwing for that service request in test code per Andrew.

[DustinCampbell]

I'm OK with this if a TODO comment is added in the code to link the issue to the hard-coded bit.

[alexgav]

@dotnet/razor-compiler ptal

[jjonescz]

compiler LGTM