Support detecting begin()/end() methods for range-for loops (#7185)

This commit adds support for range-based for loops using C++ types. It's
currently limited to detecting that `r.begin()` and `r.end()` are
available. We should be able to add full support for methods after #7181
is merged.

Support for ADL is still a work-in-progress, and will be added at a
later time.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

core/prelude/iterate.carbon

@@ -30,3 +30,10 @@ impl forall [T:! Copy & Destroy, N:! IntLiteral()]
     }
   }
 }
+
+interface CppRangeForIterate {
+  let Iterator:! type;
+  let Sentinel:! type;
+  fn Begin[ref self: Self]() -> Iterator;
+  fn End[ref self: Self]() -> Sentinel;
+}