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/dead_code_elimination.cc b/compiler/optimizing/dead_code_elimination.cc
index ee868db..909ff83 100644
--- a/compiler/optimizing/dead_code_elimination.cc
+++ b/compiler/optimizing/dead_code_elimination.cc
@@ -25,7 +25,7 @@
 #include "optimizing/nodes.h"
 #include "ssa_phi_elimination.h"
 
-namespace art {
+namespace art HIDDEN {
 
 static void MarkReachableBlocks(HGraph* graph, ArenaBitVector* visited) {
   // Use local allocator for allocating memory.