Fix WrapWithSafeLetCallFix ignore extension method

Related to KT-17726

idea/testData/quickfix/replaceWithSafeCall/noReplaceWithSafeCallForImplicitReceiver.kt

@@ -2,7 +2,6 @@
 // ACTION: Add non-null asserted (!!) call
 // ACTION: Convert to expression body
 // ACTION: Replace with safe (this?.) call
-// ACTION: Wrap with '?.let { ... }' call
 // ERROR: Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type A?
 
 class A {

idea/testData/quickfix/wrapWithSafeLetCall/extensionMethod.kt

@@ -0,0 +1,10 @@
+// "Wrap with '?.let { ... }' call" "false"
+// WITH_RUNTIME
+// ACTION: Add non-null asserted (!!) call
+// ACTION: Introduce local variable
+// ACTION: Replace with safe (this?.) call
+// ERROR: Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type String?
+
+fun String?.foo() {
+    toLowerCase<caret>()
+}