[Sema] HLSLCompareFloatingRank - don't dereference getAs<> results

Use castAs<> to assert the cast is valid to help avoid null dereferences

Fixes static analyser warning

Author: RKSimon

clang/lib/Sema/SemaOverload.cpp

@@ -4372,8 +4372,8 @@ HLSLCompareFloatingRank(QualType LHS, QualType RHS) {
   if (const auto *VT = RHS->getAs<VectorType>())
     RHS = VT->getElementType();
 
-  const auto L = LHS->getAs<BuiltinType>()->getKind();
-  const auto R = RHS->getAs<BuiltinType>()->getKind();
+  const auto L = LHS->castAs<BuiltinType>()->getKind();
+  const auto R = RHS->castAs<BuiltinType>()->getKind();
   if (L == R)
     return ImplicitConversionSequence::Indistinguishable;
   return L < R ? ImplicitConversionSequence::Better