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/graph_visualizer.cc b/compiler/optimizing/graph_visualizer.cc
index 601bebd..9419c8d 100644
--- a/compiler/optimizing/graph_visualizer.cc
+++ b/compiler/optimizing/graph_visualizer.cc
@@ -40,7 +40,7 @@
 #include "ssa_liveness_analysis.h"
 #include "utils/assembler.h"
 
-namespace art HIDDEN {
+namespace art {
 
 static bool HasWhitespace(const char* str) {
   DCHECK(str != nullptr);