Implement support for mixed-access overload sets. (#6137)

This turns out to be quite important, as several important standard
library types (such as `std::string`) have mixed-access overload sets
for their constructors as an implementation detail. The overall approach
here is:

- Use the most permissive access to determine the access of the overload
set itself. This affects whether name lookup finds the member name at
all.
- After overload resolution, re-check the access of the selected member,
if it's protected or private.

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>

toolchain/check/cpp/operators.cpp

@@ -206,7 +206,8 @@ auto LookupCppOperator(Context& context, SemIR::LocId loc_id, Operator op,
   }
 
   return ImportCppOverloadSet(context, SemIR::NameScopeId::None,
-                              SemIR::NameId::CppOperator, functions);
+                              SemIR::NameId::CppOperator,
+                              /*naming_class=*/nullptr, std::move(functions));
 }
 
 }  // namespace Carbon::Check