hash_generator_testing: Avoid using invalid enum values

Invalid enum values are undefined behavior. This change
makes the enum backed by an integer for testing.

Our UBSAN tests were missing `-fno-sanitize-recover`, which
means UBSAN logs a warning, but the program continues,
causing the test not to fail.

`-fno-sanitize-recover` will be added once all errors are fixed.

PiperOrigin-RevId: 733445015
Change-Id: Ie31fe1d06520d91dfe291b4488fe3d4ca29c72c0

Author: derekmauro

absl/container/internal/hash_generator_testing.h

@@ -50,7 +50,7 @@ struct IsMap<Map, absl::void_t<typename Map::mapped_type>> : std::true_type {};
 
 std::mt19937_64* GetSharedRng();
 
-enum Enum {
+enum Enum : uint64_t {
   kEnumEmpty,
   kEnumDeleted,
 };
@@ -80,11 +80,7 @@ struct Generator<Enum> {
   Enum operator()() const {
     std::uniform_int_distribution<typename std::underlying_type<Enum>::type>
         dist;
-    while (true) {
-      auto variate = dist(*GetSharedRng());
-      if (variate != kEnumEmpty && variate != kEnumDeleted)
-        return static_cast<Enum>(variate);
-    }
+    return static_cast<Enum>(dist(*GetSharedRng()));
   }
 };
 
@@ -94,11 +90,7 @@ struct Generator<EnumClass> {
     std::uniform_int_distribution<
         typename std::underlying_type<EnumClass>::type>
         dist;
-    while (true) {
-      EnumClass variate = static_cast<EnumClass>(dist(*GetSharedRng()));
-      if (variate != EnumClass::kEmpty && variate != EnumClass::kDeleted)
-        return static_cast<EnumClass>(variate);
-    }
+    return static_cast<EnumClass>(dist(*GetSharedRng()));
   }
 };