util/ribbon_test.cc: avoid ambiguous reversed operator error in c++20 (facebook#11371)

Summary:
util/ribbon_test.cc: avoid ambiguous reversed operator error in c++20 (and enable checking for the error)

Code would produce errors like this, when compiled with -Wambiguous-reversed-operator under c++20.
```
util/ribbon_test.cc:695:20: error: ISO C++20 considers use of overloaded operator '!=' (with operand types 'KeyGen' (aka '(anonymous namespace)::StandardKeyGen') and 'KeyGen') to be ambiguou
s despite there being a unique best viable function with non-reversed arguments [-Werror,-Wambiguous-reversed-operator]
        while (cur != batch_end) {
               ~~~ ^  ~~~~~~~~~
util/ribbon_test.cc:111:8: note: candidate function with non-reversed arguments
  bool operator!=(const StandardKeyGen& other) {
       ^
util/ribbon_test.cc:107:8: note: ambiguous candidate function with reversed arguments
  bool operator==(const StandardKeyGen& other) {
       ^
```

This will become a hard error in future standards.

Confirmed that no errors were generated when building using clang and c++20:
```
USE_CLANG=1 USE_COROUTINES=1 make
```

Pull Request resolved: facebook#11371

Reviewed By: meyering

Differential Revision: D44921027

Pulled By: cbi42

fbshipit-source-id: ef25b78260920a4d75a718310688d3a2487ffa87

Author: Jeff Palm

Makefile

@@ -539,7 +539,7 @@ endif
 
 ifdef USE_CLANG
 	# Used by some teams in Facebook
-	WARNING_FLAGS += -Wshift-sign-overflow
+	WARNING_FLAGS += -Wshift-sign-overflow -Wambiguous-reversed-operator
 endif
 
 ifeq ($(PLATFORM), OS_OPENBSD)

util/ribbon_test.cc

@@ -104,11 +104,11 @@ struct StandardKeyGen {
     return str_;
   }
 
-  bool operator==(const StandardKeyGen& other) {
+  bool operator==(const StandardKeyGen& other) const {
     // Same prefix is assumed
     return id_ == other.id_;
   }
-  bool operator!=(const StandardKeyGen& other) {
+  bool operator!=(const StandardKeyGen& other) const {
     // Same prefix is assumed
     return id_ != other.id_;
   }
@@ -144,8 +144,8 @@ struct SmallKeyGen {
     return str_;
   }
 
-  bool operator==(const SmallKeyGen& other) { return id_ == other.id_; }
-  bool operator!=(const SmallKeyGen& other) { return id_ != other.id_; }
+  bool operator==(const SmallKeyGen& other) const { return id_ == other.id_; }
+  bool operator!=(const SmallKeyGen& other) const { return id_ != other.id_; }
 
   uint64_t id_;
   std::string str_;
@@ -1069,11 +1069,11 @@ struct PhsfInputGen {
 
   const std::pair<std::string, uint8_t>* operator->() { return &**this; }
 
-  bool operator==(const PhsfInputGen& other) {
+  bool operator==(const PhsfInputGen& other) const {
     // Same prefix is assumed
     return id_ == other.id_;
   }
-  bool operator!=(const PhsfInputGen& other) {
+  bool operator!=(const PhsfInputGen& other) const {
     // Same prefix is assumed
     return id_ != other.id_;
   }