Always wrap classes with decorators or static properties in an IIFE

Currently only script targets less than or equal to ES5 will wrap classes.  However, the wrapping is also crucial to file size optimizations for ES2015+ as well.  Without the IIFE wrapper, minification tools do not elide the class.  This is due to references to the class being present within the downlevelled decorator and static property code.
This change represents the full completion of issue #15857

Author: clydin

src/compiler/transformers/ts.ts

@@ -23,11 +23,10 @@ namespace ts {
         IsNamedExternalExport = 1 << 4,
         IsDefaultExternalExport = 1 << 5,
         IsDerivedClass = 1 << 6,
-        UseImmediatelyInvokedFunctionExpression = 1 << 7,
 
         HasAnyDecorators = HasConstructorDecorators | HasMemberDecorators,
         NeedsName = HasStaticInitializedProperties | HasMemberDecorators,
-        MayNeedImmediatelyInvokedFunctionExpression = HasAnyDecorators | HasStaticInitializedProperties,
+        UseImmediatelyInvokedFunctionExpression = HasAnyDecorators | HasStaticInitializedProperties,
         IsExported = IsExportOfNamespace | IsDefaultExternalExport | IsNamedExternalExport,
     }
 
@@ -587,7 +586,6 @@ namespace ts {
             if (isExportOfNamespace(node)) facts |= ClassFacts.IsExportOfNamespace;
             else if (isDefaultExternalModuleExport(node)) facts |= ClassFacts.IsDefaultExternalExport;
             else if (isNamedExternalModuleExport(node)) facts |= ClassFacts.IsNamedExternalExport;
-            if (languageVersion <= ScriptTarget.ES5 && (facts & ClassFacts.MayNeedImmediatelyInvokedFunctionExpression)) facts |= ClassFacts.UseImmediatelyInvokedFunctionExpression;
             return facts;
         }
 
@@ -666,7 +664,7 @@ namespace ts {
                             /*type*/ undefined,
                             iife
                         )
-                    ])
+                    ], languageVersion > ScriptTarget.ES5 ? NodeFlags.Let : undefined)
                 );
 
                 setOriginalNode(varStatement, node);