Revert "Make compiler/optimizing/ symbols hidden."

This reverts commit e2727154f25e0db9a5bb92af494d8e47b181dfcf.

Reason for revert: Breaks ASAN tests (ODR violation).
Bug: 142365358

Change-Id: I38103d74a1297256c81d90872b6902ff1e9ef7a4
diff --git a/compiler/optimizing/gvn.cc b/compiler/optimizing/gvn.cc
index 8d46286..e8460a8 100644
--- a/compiler/optimizing/gvn.cc
+++ b/compiler/optimizing/gvn.cc
@@ -23,7 +23,7 @@
 #include "base/utils.h"
 #include "side_effects_analysis.h"
 
-namespace art HIDDEN {
+namespace art {
 
 /**
  * A ValueSet holds instructions that can replace other instructions. It is updated