[CS] Add a fallback for overload locators that failed to simplify

If an overload locator couldn't be simplified to its anchor, fallback to
the anchor of the locator instead. Also add an assert so that we can
track down whether this is actually valid or not.

Resolves rdar://89097800.

Author: bnbarham

lib/Sema/ConstraintSystem.cpp

@@ -4483,6 +4483,15 @@ bool ConstraintSystem::diagnoseAmbiguity(ArrayRef<Solution> solutions) {
     // is not sufficiently centralized in the AST.
     DeclNameRef name(getOverloadChoiceName(overload.choices));
     auto anchor = simplifyLocatorToAnchor(overload.locator);
+    if (!anchor) {
+      // It's not clear that this is actually valid. Just use the overload's
+      // anchor for release builds, but assert so we can properly diagnose
+      // this case if it happens to be hit. Note that the overload will
+      // *always* be anchored, otherwise everything would be broken, ie. this
+      // assertion would be the least of our worries.
+      anchor = overload.locator->getAnchor();
+      assert(false && "locator could not be simplified to anchor");
+    }
 
     // Emit the ambiguity diagnostic.
     auto &DE = getASTContext().Diags;