Simplify CSharpRequiredLanguageVersion

[Youssef1313]

LanguageVersion.Preview.MapSpecifiedToEffectiveVersion() will always return "Preview" as far as I understand. See:

roslyn/src/Compilers/CSharp/Portable/LanguageVersion.cs

Lines 370 to 384 in 6da1274

	/// <summary>
	/// Map a language version (such as Default, Latest, or CSharpN) to a specific version (CSharpM).
	/// </summary>
	public static LanguageVersion MapSpecifiedToEffectiveVersion(this LanguageVersion version)
	{
	switch (version)
	{
	case LanguageVersion.Latest:
	case LanguageVersion.Default:
	case LanguageVersion.LatestMajor:
	return LanguageVersion.CSharp9;
	default:
	return version;
	}
	}

Thus, the Version property is always set to version.

Did Preview in past used to map to a language version?

Could this be taken further and simplify all new CSharpRequiredLanguageVersion(...), and even trying to get rid of the class (if possible)? (Probably not possible - See string? GetRequiredLanguageVersion(Diagnostic diagnostic) - But I'll give it a try later)

[cston]

Thanks @Youssef1313.