Don't provide string literal completions for string enums (#18288)

* Don't provide string literal completions for string enums

* Rename test

Author: Andy

src/services/completions.ts

@@ -284,7 +284,7 @@ namespace ts.Completions {
                 addStringLiteralCompletionsFromType(t, result, typeChecker, uniques);
             }
         }
-        else if (type.flags & TypeFlags.StringLiteral) {
+        else if (type.flags & TypeFlags.StringLiteral && !(type.flags & TypeFlags.EnumLiteral)) {
             const name = (<StringLiteralType>type).value;
             if (!uniques.has(name)) {
                 uniques.set(name, true);

tests/cases/fourslash/stringLiteralCompletionsForStringEnumContextualType.ts

@@ -0,0 +1,9 @@
+/// <reference path="fourslash.ts" />
+
+////const enum E {
+////    A = "A",
+////}
+////const e: E = "/**/";
+
+goTo.marker("");
+verify.completionListIsEmpty();