Improve Guid v7 performance on Unix

src/libraries/System.Private.CoreLib/src/System/Guid.Unix.cs

@@ -33,5 +33,22 @@ public static unsafe Guid NewGuid()
 
             return g;
         }
+
+        private static unsafe Guid CreateRandomizedPartialVersion7()
+        {
+            Guid g;
+
+            byte* randomDataStartOffset = ((byte*)&g) + 6;
+            int randomDataLength = sizeof(Guid) - 6;
+
+#if !TARGET_WASI
+            Interop.GetCryptographicallySecureRandomBytes(randomDataStartOffset, randomDataLength);
+#else
+            // TODOWASI: crypto secure random bytes
+            Interop.GetRandomBytes(randomDataStartOffset, randomDataLength);
+#endif
+
+            return g;
+        }
     }
 }

src/libraries/System.Private.CoreLib/src/System/Guid.Windows.cs

@@ -26,5 +26,12 @@ private static void ThrowForHr(int hr)
             // we ever hit this condition.
             throw new Exception() { HResult = hr };
         }
+
+        private static Guid CreateRandomizedPartialVersion7()
+        {
+            // CoCreateGuid (above) is faster than Interop.BCrypt.BCryptGenRandom and does more than what we need, so
+            // we may as well use that here.
+            return NewGuid();
+        }
     }
 }

src/libraries/System.Private.CoreLib/src/System/Guid.cs

@@ -303,12 +303,11 @@ public Guid(string g)
         /// </remarks>
         public static Guid CreateVersion7(DateTimeOffset timestamp)
         {
-            // NewGuid uses CoCreateGuid on Windows and Interop.GetCryptographicallySecureRandomBytes on Unix to get
-            // cryptographically-secure random bytes. We could use Interop.BCrypt.BCryptGenRandom to generate the random
-            // bytes on Windows, as is done in RandomNumberGenerator, but that's measurably slower than using CoCreateGuid.
-            // And while CoCreateGuid only generates 122 bits of randomness, the other 6 bits being for the version / variant
-            // fields, this method also needs those bits to be non-random, so we can just use NewGuid for efficiency.
-            Guid result = NewGuid();
+            // Version 7 contains two major components: a timestamp, and randomized data.
+            // This function call creates a GUID with _at least_ the randomized data set.
+            // On Windows this creates a full Version 4 GUID, as it is faster than just getting random data.
+            // On Unix this only fills the random data bytes and leaves the rest unset.
+            Guid result = CreateRandomizedPartialVersion7();
 
             // 2^48 is roughly 8925.5 years, which from the Unix Epoch means we won't
             // overflow until around July of 10,895. So there isn't any need to handle