Merge #31: Use memory_order symbols consistently

93ee8d8 Merge pull request google#965 from ShawnZhong:cpp20 (Victor Costan)

Pull request description:

  This is cherry-picked from upstream and can be dropped when upstream is fully synced to include the commit.

  For now this is needed for two reasons:

  * Consistently use the same symbol names in the whole project.
  * Compiling with C++20 fails.

ACKs for top commit:
  achow101:
    ACK 93ee8d8

Tree-SHA512: 3ab48f003cd01ff9dce2da8d68085fa5ff100c3e6de75a0a4b4dd1b669d78577c85d7663906441a410d8c96965fc059a574ff6ef9b2741f9a49bd31d65acb896

util/env_posix.cc

@@ -850,7 +850,7 @@ class SingletonEnv {
  public:
   SingletonEnv() {
 #if !defined(NDEBUG)
-    env_initialized_.store(true, std::memory_order::memory_order_relaxed);
+    env_initialized_.store(true, std::memory_order_relaxed);
 #endif  // !defined(NDEBUG)
     static_assert(sizeof(env_storage_) >= sizeof(EnvType),
                   "env_storage_ will not fit the Env");
@@ -867,7 +867,7 @@ class SingletonEnv {
 
   static void AssertEnvNotInitialized() {
 #if !defined(NDEBUG)
-    assert(!env_initialized_.load(std::memory_order::memory_order_relaxed));
+    assert(!env_initialized_.load(std::memory_order_relaxed));
 #endif  // !defined(NDEBUG)
   }
 

util/env_windows.cc

@@ -798,7 +798,7 @@ class SingletonEnv {
  public:
   SingletonEnv() {
 #if !defined(NDEBUG)
-    env_initialized_.store(true, std::memory_order::memory_order_relaxed);
+    env_initialized_.store(true, std::memory_order_relaxed);
 #endif  // !defined(NDEBUG)
     static_assert(sizeof(env_storage_) >= sizeof(EnvType),
                   "env_storage_ will not fit the Env");
@@ -815,7 +815,7 @@ class SingletonEnv {
 
   static void AssertEnvNotInitialized() {
 #if !defined(NDEBUG)
-    assert(!env_initialized_.load(std::memory_order::memory_order_relaxed));
+    assert(!env_initialized_.load(std::memory_order_relaxed));
 #endif  // !defined(NDEBUG)
   }