[CS] Allow getCalleeLocator to find key path component callees

In order to do this we need it to take a ConstraintLocator argument so
we can tell which component we want the callee for. To make it clear
that we're looking for a callee at the anchor, also rename the member
to getAnchormostCalleeLocator.

lib/Sema/CSApply.cpp

@@ -4243,11 +4243,11 @@ namespace {
 
         auto locator = cs.getConstraintLocator(
             E, LocatorPathElt::KeyPathComponent(i));
-        if (kind == KeyPathExpr::Component::Kind::UnresolvedSubscript) {
-          locator =
-            cs.getConstraintLocator(locator,
-                                    ConstraintLocator::SubscriptMember);
-        }
+
+        // Adjust the locator such that it includes any additional elements to
+        // point to the component's callee, e.g a SubscriptMember for a
+        // subscript component.
+        locator = cs.getAnchormostCalleeLocator(locator);
 
         bool isDynamicMember = false;
         // If this is an unresolved link, make sure we resolved it.