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/codegen_test.cc b/compiler/optimizing/codegen_test.cc
index 9949ef8..26d07bd 100644
--- a/compiler/optimizing/codegen_test.cc
+++ b/compiler/optimizing/codegen_test.cc
@@ -35,7 +35,7 @@
 
 #include "gtest/gtest.h"
 
-namespace art HIDDEN {
+namespace art {
 
 // Return all combinations of ISA and code generator that are executable on
 // hardware, or on simulator, and that we'd like to test.