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.cc b/compiler/optimizing/code_generator_x86.cc
index 9e066b7..797fe32 100644
--- a/compiler/optimizing/code_generator_x86.cc
+++ b/compiler/optimizing/code_generator_x86.cc
@@ -37,7 +37,7 @@
 #include "utils/x86/assembler_x86.h"
 #include "utils/x86/managed_register_x86.h"
 
-namespace art HIDDEN {
+namespace art {
 
 template<class MirrorType>
 class GcRoot;