When creating string literal for property access, unescape the identifier text

Fixes #12562

src/compiler/transformers/ts.ts

@@ -1905,7 +1905,7 @@ namespace ts {
                     : (<ComputedPropertyName>name).expression;
             }
             else if (isIdentifier(name)) {
-                return createLiteral(name.text);
+                return createLiteral(unescapeIdentifier(name.text));
             }
             else {
                 return getSynthesizedClone(name);

tests/baselines/reference/decoratorWithUnderscoreMethod.js

@@ -34,4 +34,4 @@ var A = (function () {
 }());
 __decorate([
     dec()
-], A.prototype, "___foo");
+], A.prototype, "__foo");