[CS] Don’t crash if locator passed to hasPreconcurrencyCallee has no anchor

When computing type relations of code completion items with respect to the context they are used in, we add a `ConstraintKind::Conversion` constraint with an empty constraint locator. This currently crashes in `hasPreconcurrencyCallee`.

To fix this crash, just return `false` in these cases.

Author: ahoppen

lib/Sema/CSSimplify.cpp

@@ -2657,7 +2657,11 @@ static bool fixExtraneousArguments(ConstraintSystem &cs,
 
 bool ConstraintSystem::hasPreconcurrencyCallee(
     ConstraintLocatorBuilder locator) {
-  auto calleeLocator = getCalleeLocator(getConstraintLocator(locator));
+  auto resolvedLocator = getConstraintLocator(locator);
+  if (resolvedLocator->getAnchor().isNull()) {
+    return false;
+  }
+  auto calleeLocator = getCalleeLocator(resolvedLocator);
   auto calleeOverload = findSelectedOverloadFor(calleeLocator);
   if (!calleeOverload || !calleeOverload->choice.isDecl())
     return false;