SemaExpr.cpp - use castAs<> instead of getAs<> as we dereference the pointer directly. NFCI.

castAs<> will assert the correct cast type instead of just returning null, which we then try to dereference immediately.

clang/lib/Sema/SemaExpr.cpp

@@ -15166,7 +15166,7 @@ void Sema::ActOnBlockArguments(SourceLocation CaretLoc, Declarator &ParamInfo,
   CurBlock->TheDecl->setSignatureAsWritten(Sig);
   CurBlock->FunctionType = T;
 
-  const FunctionType *Fn = T->getAs<FunctionType>();
+  const auto *Fn = T->castAs<FunctionType>();
   QualType RetTy = Fn->getReturnType();
   bool isVariadic =
       (isa<FunctionProtoType>(Fn) && cast<FunctionProtoType>(Fn)->isVariadic());