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/nodes_vector.h b/compiler/optimizing/nodes_vector.h
index 5cc1b3c..efe4d6b 100644
--- a/compiler/optimizing/nodes_vector.h
+++ b/compiler/optimizing/nodes_vector.h
@@ -21,7 +21,7 @@
 // is included in the header file nodes.h itself. However it gives editing tools better context.
 #include "nodes.h"
 
-namespace art HIDDEN {
+namespace art {
 
 // Memory alignment, represented as an offset relative to a base, where 0 <= offset < base,
 // and base is a power of two. For example, the value Alignment(16, 0) means memory is