Report WRONG_MODIFIER_TARGET on the modifier instead of the declaration

in ExternalFunChecker.

This allows RemoveModifierFix to provide a quickfix to remove it.

idea/testData/quickfix/modifiers/removeExternalModifier.kt

@@ -0,0 +1,5 @@
+// "Remove 'external' modifier" "true"
+
+class B {
+    <caret>external val foo: Int = 23
+}