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/linear_order.cc b/compiler/optimizing/linear_order.cc
index 25ca866..58e00a8 100644
--- a/compiler/optimizing/linear_order.cc
+++ b/compiler/optimizing/linear_order.cc
@@ -19,7 +19,7 @@
 #include "base/scoped_arena_allocator.h"
 #include "base/scoped_arena_containers.h"
 
-namespace art HIDDEN {
+namespace art {
 
 static bool InSameLoop(HLoopInformation* first_loop, HLoopInformation* second_loop) {
   return first_loop == second_loop;