ART: Add -verbose:dex

Add a dex verbose flag to separate out concerns for dex investigations.

Bug: 37158297
Test: m test-art-host
Change-Id: Ice5efad919ae0b6838d44e2f3711563a0774ef0e
diff --git a/runtime/base/logging.h b/runtime/base/logging.h
index a173ac2..7a9184e 100644
--- a/runtime/base/logging.h
+++ b/runtime/base/logging.h
@@ -56,6 +56,7 @@
   bool image;
   bool systrace_lock_logging;  // Enabled with "-verbose:sys-locks".
   bool agents;
+  bool dex;  // Some dex access output etc.
 };
 
 // Global log verbosity setting, initialized by InitLogging.