add getStringValue() to value resolver (#1130)

src/PhpParser/Node/Value/ValueResolver.php

@@ -49,6 +49,16 @@ final class ValueResolver
         return $this->getValue($expr) === $value;
     }
 
+    public function getStringValue(Expr $expr): string
+    {
+        $resolvedValue = $this->getValue($expr);
+        if (! is_string($resolvedValue)) {
+            throw new ShouldNotHappenException();
+        }
+
+        return $resolvedValue;
+    }
+
     /**
      * @return mixed|null
      */