supported language version when used in APIs.
Also update the C# 7 feature status doc to bring it up-to-date.

We also change the default language version for VB from 14 to 15, and
put the binary literals and digit separators under language version 15.

This PR takes the approach of immediately transforming a "Latest"
language version to the actual latest language version. Thus, the extra enum value
should never be persisted as part of an options object, but only transiently
appear as an argument to an API call setting or resetting the language version.

- Feature flag.
- Syntax model changes.
- Parsing.
- Binding (for explicitly typed variables only).
- Flow analysis.
- SemanticModel – added new API GetDeclaredSymbol(ArgumentSyntax) to return symbol for the declared local.

Basic scenarios work end to end.

Since "original" is by itself contextual on the presence of "replace", only "replace" actually needs to be keyed off a flag.

Tuples: adding feature flag

and for each one, either
1. address the issue, or
2. ensure that there is an issue tracking it

Fixes #9659 #9662

add __DEMO__ pp-symbol to enable all experimental features

Fell out of our normal cadence for this during the push for RTM. Now that we
had a bit more breathing room getting us back on track here.

This reverts 99317ad0.

Fixes #2725

The principal change here is to remove the language version "experimenal." However, there are a number of places in the code where we either set the language version to experimental or check whether experimental features are enabled. Those are largely vestigal at the moment as we have no experimental features. However as soon as we add experimental features that code will once again be useful. Rather than deleting that code now, I wanted to leave it in and make it easy to find and update as features are added. So I'm adding a stubbed implementation of the API proposed for future use for enabling experimental features.

After Dev14 our plan is to have experimental features enabled one-by-one rather than all under a single umbrella "experimental" flag. Since language features affect the parser, this is part of the parse options. This changeset adds a stubbed version of the proposed APIs:

public class ParseOptions
{
    /// <summary>
    /// Enable some experimental language features for testing.
    /// </summary>
    public ParseOptions WithFeatures(IEnumerable<KeyValuePair<string, string>> features);

    /// <summary>
    /// Returns the experimental features.
    /// </summary>
    public abstract IReadOnlyDictionary<string, string> Features
    {
        get;
    }
} (changeset 1369555)

Splits Microsoft.CodeAnalysis, Microsoft.CodeAnalysis.CSharp and Microsoft.CodeAnalysis.VisualBasic into portable and desktop assemblies.

Existing Core\Source, CSharp\Source and VisualBasic\Source directories are renamed to Core\Portable, CSharp\Portable and VisualBasic\Portable.
New sibling Desktop folders are added and non-portable source is moved there.
"System.Runtime" references has to be removed in order for the portable project system magic to automatically add facade references.
 (changeset 1281686)

Move serialization from CompilationOptions and ParseOptions to dedicated SerializableCompilationOptions and SerializableParseOptions wrappers. (changeset 1274332)