Making the deleted copy constructor parameter const; NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@254520 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Sema/AttributeList.h

@@ -557,7 +557,7 @@ public:
   /// Create a new pool for a factory.
   AttributePool(AttributeFactory &factory) : Factory(factory), Head(nullptr) {}
 
-  AttributePool(AttributePool &) = delete;
+  AttributePool(const AttributePool &) = delete;
 
   /// Move the given pool's allocations to this pool.
   AttributePool(AttributePool &&pool) : Factory(pool.Factory), Head(pool.Head) {