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

[vinograd47]

Summary

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

[vinograd47]

Fixed clang-tidy issues

[vinograd47]

Rebased

llvm/include/llvm/ADT/STLExtras.h

indexed_accessor_range_base(iterator begin, iterator end)
         count(end.getIndex() - begin.getIndex()) {}
   indexed_accessor_range_base(const iterator_range<iterator> &range)
       : indexed_accessor_range_base(range.begin(), range.end()) {}
   indexed_accessor_range_base(BaseT base, ptrdiff_t count)
       : base(base), count(count) {}
 
   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");
     return (*this)[0];
   }
   ReferenceT back() const {
     assert(!empty() && "expected non-empty range");
     return (*this)[size() - 1];