[llvm] Fixed build with C++20 standard (PR #169772)

[A. Jiang via llvm-commits]

================
@@ -267,7 +267,11 @@ class iterator_adaptor_base
     return *static_cast<DerivedT *>(this);
   }
   using BaseT::operator-;
-  difference_type operator-(const DerivedT &RHS) const {
+  difference_type operator-(const DerivedT &RHS) const
+#ifdef __cpp_concepts
+    requires(bool(BaseT::IsRandomAccess))
+#endif
+  {
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frederick-vs-ja wrote:

How about also constrain this in C++17?
```suggestion
  template <bool Enabled = bool(BaseT::IsRandomAccess), std::enable_if_t<Enabled, int> = 0>
  difference_type operator-(const DerivedT &RHS) const {
```

https://github.com/llvm/llvm-project/pull/169772