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/loop_optimization.cc b/compiler/optimizing/loop_optimization.cc
index 4661d73..9c4e9d2 100644
--- a/compiler/optimizing/loop_optimization.cc
+++ b/compiler/optimizing/loop_optimization.cc
@@ -28,7 +28,7 @@
 #include "mirror/array-inl.h"
 #include "mirror/string.h"
 
-namespace art HIDDEN {
+namespace art {
 
 // Enables vectorization (SIMDization) in the loop optimizer.
 static constexpr bool kEnableVectorization = true;