Simplify isExpression check (microsoft#17741)

Author: Andy

src/compiler/checker.ts

@@ -15220,7 +15220,7 @@ namespace ts {
             // example, given a 'function wrap<T, U>(cb: (x: T) => U): (x: T) => U' and a call expression
             // 'let f: (x: string) => number = wrap(s => s.length)', we infer from the declared type of 'f' to the
             // return type of 'wrap'.
-            if (isExpression(node)) {
+            if (node.kind !== SyntaxKind.Decorator) {
                 const contextualType = getContextualType(node);
                 if (contextualType) {
                     // We clone the contextual mapper to avoid disturbing a resolution in progress for an