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_mips64.cc b/compiler/optimizing/code_generator_mips64.cc
index 9eff30d..75bca4e 100644
--- a/compiler/optimizing/code_generator_mips64.cc
+++ b/compiler/optimizing/code_generator_mips64.cc
@@ -38,7 +38,7 @@
 #include "utils/mips64/assembler_mips64.h"
 #include "utils/stack_checks.h"
 
-namespace art HIDDEN {
+namespace art {
 namespace mips64 {
 
 static constexpr int kCurrentMethodStackOffset = 0;