Reland "Make compiler/optimizing/ symbols hidden."

This reverts commit 0a51605ddd81635135463dab08b6f7c21b58ffb0.

Reason for revert: Reland after some of the required work
was merged in other CLs.

Also address a TODO from the original CL to mark required
symbols with EXPORT in `intrinsic_objects.h`.

Also mark symbols in new files as HIDDEN.

Bug: 186902856
Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I936d448983928af23614ca82c2d0bf9a645e2c52
diff --git a/compiler/optimizing/loop_optimization.h b/compiler/optimizing/loop_optimization.h
index 0535c74..6dd778b 100644
--- a/compiler/optimizing/loop_optimization.h
+++ b/compiler/optimizing/loop_optimization.h
@@ -17,6 +17,7 @@
 #ifndef ART_COMPILER_OPTIMIZING_LOOP_OPTIMIZATION_H_
 #define ART_COMPILER_OPTIMIZING_LOOP_OPTIMIZATION_H_
 
+#include "base/macros.h"
 #include "base/scoped_arena_allocator.h"
 #include "base/scoped_arena_containers.h"
 #include "induction_var_range.h"
@@ -25,7 +26,7 @@
 #include "optimization.h"
 #include "superblock_cloner.h"
 
-namespace art {
+namespace art HIDDEN {
 
 class CompilerOptions;
 class ArchNoOptsLoopHelper;