[ADT][NFC] Use size_t type for index in indexed_accessor_range

It makes it consistent with `size()` method return type and with
STL-like containers API.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D97921

llvm/include/llvm/ADT/STLExtras.h

@@ -1114,9 +1114,9 @@ class indexed_accessor_range_base {
 
   iterator begin() const { return iterator(base, 0); }
   iterator end() const { return iterator(base, count); }
-  ReferenceT operator[](unsigned index) const {
-    assert(index < size() && "invalid index for value range");
-    return DerivedT::dereference_iterator(base, index);
+  ReferenceT operator[](size_t Index) const {
+    assert(Index < size() && "invalid index for value range");
+    return DerivedT::dereference_iterator(base, static_cast<ptrdiff_t>(Index));
   }
   ReferenceT front() const {
     assert(!empty() && "expected non-empty range");