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/code_generator_x86_64.cc b/compiler/optimizing/code_generator_x86_64.cc
index e3b4e40..2d51f32 100644
--- a/compiler/optimizing/code_generator_x86_64.cc
+++ b/compiler/optimizing/code_generator_x86_64.cc
@@ -37,7 +37,7 @@
 #include "utils/x86_64/assembler_x86_64.h"
 #include "utils/x86_64/managed_register_x86_64.h"
 
-namespace art HIDDEN {
+namespace art {
 
 template<class MirrorType>
 class GcRoot;