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_x86.h b/compiler/optimizing/nodes_x86.h
index e246390..8e8fbc1 100644
--- a/compiler/optimizing/nodes_x86.h
+++ b/compiler/optimizing/nodes_x86.h
@@ -17,7 +17,7 @@
 #ifndef ART_COMPILER_OPTIMIZING_NODES_X86_H_
 #define ART_COMPILER_OPTIMIZING_NODES_X86_H_
 
-namespace art HIDDEN {
+namespace art {
 
 // Compute the address of the method for X86 Constant area support.
 class HX86ComputeBaseMethodAddress final : public HExpression<0> {