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/debug/method_debug_info.h b/compiler/debug/method_debug_info.h
index 729c403..152db6e 100644
--- a/compiler/debug/method_debug_info.h
+++ b/compiler/debug/method_debug_info.h
@@ -32,7 +32,7 @@
   size_t class_def_index;
   uint32_t dex_method_index;
   uint32_t access_flags;
-  const DexFile::CodeItem* code_item;
+  const dex::CodeItem* code_item;
   InstructionSet isa;
   bool deduped;
   bool is_native_debuggable;