Address llvm::PointerUnion::{is,get} deprecations

These were deprecated in
https://github.com/llvm/llvm-project/pull/122623.

lib/Sema/ConstraintGraph.cpp

@@ -810,7 +810,7 @@ namespace {
 
       for (auto typeVar : typeVars) {
         auto &impl = typeVar->getImpl();
-        if (impl.getRepresentativeOrFixed().is<TypeBase *>()) {
+        if (isa<TypeBase *>(impl.getRepresentativeOrFixed())) {
           auto &node = cg[typeVar];
           for (auto otherTypeVar : node.getReferencedVars()) {
             unionSets(typeVar, otherTypeVar);