Add 'CSharpVersion7' member into public 'Language' enum (#3933)

Public enum 'LanguageVersion' from 'Microsoft.CodeAnalysis.CSharp'
already contains 'CSharp7' member.
After moving PowerShell to .Net Core 2.0 we should add 'CSharpVersion7'
member into public 'Microsoft.PowerShell.Commands.Language' enum too.

This will allow 'Add-Type -Language CSharpVersion7'

src/Microsoft.PowerShell.Commands.Utility/commands/utility/AddType.cs

@@ -48,6 +48,11 @@ namespace Microsoft.PowerShell.Commands
         CSharp,
 
 #if CORECLR
+        /// <summary>
+        /// The C# programming language v7
+        /// </summary>
+        CSharpVersion7,
+
         /// <summary>
         /// The C# programming language v6
         /// </summary>
@@ -633,6 +638,7 @@ namespace Microsoft.PowerShell.Commands
                 case Language.CSharpVersion4:
                 case Language.CSharpVersion5:
                 case Language.CSharpVersion6:
+                case Language.CSharpVersion7:
 #endif
                     return true;
                 default:
@@ -1217,6 +1223,9 @@ namespace Microsoft.PowerShell.Commands
                     case Language.CSharpVersion6:
                         parseOptions = new CSharpParseOptions(LanguageVersion.CSharp6);
                         break;
+                    case Language.CSharpVersion7:
+                        parseOptions = new CSharpParseOptions(LanguageVersion.CSharp7);
+                        break;
                     case Language.CSharp:
                         parseOptions = new CSharpParseOptions();
                         break;

test/powershell/Modules/Microsoft.PowerShell.Utility/Add-Type.Tests.ps1

@@ -1,6 +1,10 @@
 $guid = [Guid]::NewGuid().ToString().Replace("-","")
 
 Describe "Add-Type" -Tags "CI" {
+    It "Public 'Language' enumeration contains all members" {
+        [Enum]::GetNames("Microsoft.PowerShell.Commands.Language") -join "," | Should Be "CSharp,CSharpVersion7,CSharpVersion6,CSharpVersion5,CSharpVersion4,CSharpVersion3,CSharpVersion2,CSharpVersion1,VisualBasic,JScript"
+    }
+
     It "Should not throw given a simple class definition" {
         { Add-Type -TypeDefinition "public static class foo { }" } | Should Not Throw
     }