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.

There is a small product change to fix https://devdiv.visualstudio.com/defaultcollection/DevDiv/_workitems?_a=edit&id=111538 and enable corresponding unit-test.

This reverts 99317ad0.

Binary serialization is not supported in CoreCLR and hence must be removed
from the subset which runs there.  This code is only used to support
CodeLens so rather than moving it around we are simply removing it
completely.  It will be relocated to CodeLens internally to continue supporting
their scenarios.

The Public API changes to DocumentId and ProjectId were necessary to
continue supporting serialization outside of the Roslyn code base.

closes #915

 (changeset 1389865)

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)

***NO_CI***
 (changeset 1356181)

Abbreviates the public VB-specific types (e.g. VisualBasicSyntaxTree -> VBSyntaxTree) in accordance with the .NET Class Library Guidelines to avoid overly lengthy names and consistent with the conventions used in other APIs (e.g. VB CodeDom). (changeset 1348499)

Replace usage of TypeCode throughout VB code base with SpecialType. TypeCode isn't available in portable profile. (changeset 1278340)

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

The value of LanguageVersion.Experimental should be higher than any possible future language version.

Note that changing an enum name or value is a breaking change. When we ship C# 7 we can't change Experimental from 7 to 8.
 (changeset 1265449)