ART: Move dex structs into own header

Separating out the structs from DexFile allows them to be forward-
declared, which reduces the need to include the dex_file header.

Bug: 119869270
Test: m
Change-Id: I32dde5a632884bca7435cd584b4a81883de2e7b4
diff --git a/compiler/optimizing/optimizing_unit_test.h b/compiler/optimizing/optimizing_unit_test.h
index 4e376b1..e5f6941 100644
--- a/compiler/optimizing/optimizing_unit_test.h
+++ b/compiler/optimizing/optimizing_unit_test.h
@@ -155,7 +155,7 @@
     void* aligned_data = GetAllocator()->Alloc(code_item_size);
     memcpy(aligned_data, &data[0], code_item_size);
     CHECK_ALIGNED(aligned_data, StandardDexFile::CodeItem::kAlignment);
-    const DexFile::CodeItem* code_item = reinterpret_cast<const DexFile::CodeItem*>(aligned_data);
+    const dex::CodeItem* code_item = reinterpret_cast<const dex::CodeItem*>(aligned_data);
 
     {
       ScopedObjectAccess soa(Thread::Current());