[NFC][CLANG] Fix issue with dereference null return value found by Coverity static analyzer tool

Reported by Coverity:

  In clang::FunctionDecl::isReplaceableGlobalAllocationFunction(std::optional<unsigned int> *, bool *): Return value of function which returns null is dereferenced without checking

  if (!IsSizedDelete && !Ty.isNull() && Ty->isEnumeralType()) {
       QualType T = Ty;
       //Condition TD, taking false branch.
      while (const auto *TD = T->getAs<TypedefType>())
        T = TD->getDecl()->getUnderlyingType();
        //returned_null: getAs returns nullptr (checked 95 out of 97 times).

      //Dereference null return value (NULL_RETURNS)
      // dereference: Dereferencing a pointer that might be nullptr T->getAs() when calling getDecl.
      IdentifierInfo *II = T->getAs<EnumType>()->getDecl()->getIdentifier();
      if (II && II->isStr("__hot_cold_t"))
        Consume();
    }

This patch uses castAs instead of getAs which will assert if the type doesn't match.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D150968