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_vector_arm64.cc b/compiler/optimizing/code_generator_vector_arm64.cc
index 71c94fe..df95c88 100644
--- a/compiler/optimizing/code_generator_vector_arm64.cc
+++ b/compiler/optimizing/code_generator_vector_arm64.cc
@@ -22,7 +22,7 @@
 
 using namespace vixl::aarch64;  // NOLINT(build/namespaces)
 
-namespace art HIDDEN {
+namespace art {
 namespace arm64 {
 
 using helpers::ARM64EncodableConstantOrRegister;