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/graph_test.cc b/compiler/optimizing/graph_test.cc
index 29af808..b5d7127 100644
--- a/compiler/optimizing/graph_test.cc
+++ b/compiler/optimizing/graph_test.cc
@@ -15,6 +15,7 @@
  */
 
 #include "base/arena_allocator.h"
+#include "base/macros.h"
 #include "builder.h"
 #include "nodes.h"
 #include "optimizing_unit_test.h"
@@ -22,7 +23,7 @@
 
 #include "gtest/gtest.h"
 
-namespace art {
+namespace art HIDDEN {
 
 class GraphTest : public OptimizingUnitTest {
  protected: