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/instruction_simplifier.cc b/compiler/optimizing/instruction_simplifier.cc
index 7c1ef30..63c5965 100644
--- a/compiler/optimizing/instruction_simplifier.cc
+++ b/compiler/optimizing/instruction_simplifier.cc
@@ -27,7 +27,7 @@
 #include "sharpening.h"
 #include "string_builder_append.h"
 
-namespace art HIDDEN {
+namespace art {
 
 // Whether to run an exhaustive test of individual HInstructions cloning when each instruction
 // is replaced with its copy if it is clonable.